Well, the thread that contains the unit/scenario should have all the info you need, and if not, it's best to ask specific questions there. However, I just typed this on another civ site, so I'll just paste it here for your reference...
A mod (or scenario) consists of two things: a .biq file (created by the civ editor) and the game files (containing new unit animations, art for cities, tech trees, text files, etc.). The .biq file should be placed in the scenarios folder of the civ version you're playing. So for example, if you're playing civ with the Conquests expansion, the file path for the biq file would be
(location of civ III - the place you told the computer to install it)/Conquests/Scenarios
Then you place the games files all in one big folder. Inside this folder, the files must be sorted into two other folders, one for art (named, aptly enough, "Art") and one for text (named "Text"). Inside these folders, the art must be sorted into units files, pedia icons, advisors, whatever requires new artwork, and the text file must be similarly sorted. If you're not sure what requires a new folder to hold the selected files, look at the core game files. The scenario folder must be arrayed like that.
That all sounds horribly complex, but thankfully 99% of the time, you don't actually need to do that step, because the creator of the scenario/mod will have done it for you. So don't lose heart if that sounds like too much work.
The folder must be named the exact same as the file name that the biq file tells the game to look in, or there will be a crash (the game can't find the animations/text/whatever it's looking for). To be absolutely certain that you name the file correctly, open civedit, and load the biq file. Open "scenario properties" (located in the Scenario drop down menu at the top) and look where it says "scenario search folders". This where the game will look for the artwork and stuff. If it's left blank, the game will use the default animations and text provided in civ3. Copy whatever is listed here, and paste it as the name of the file with all the custom stuff.
If you're downloading this scenario from someone else, and other people have played it before, this is often unnecessary, because you can be sure that the folder is already properly named. In addition, as I said above, the files will already be neatly arranged into their folder, so all you need to do is place it in the scenarios folder. The reason I gave such detailed instruction is to give you an idea of how it works, and so that you can troubleshoot on your own if you come up with any problems.
Now, open civIII, and click on civ-content. Select your mod/scenario, and you should be good to go. If you get a file error, that could mean one of several things.
-You named the folder incorrectly.
-The file specified in the biq file is incorrect.
-The file did not contain all of the information it should have.
-The file became corrupted when you downloaded it (this can happen if it's a large download and/or you have a slower connection).
Some scenarios take a very long time to load the first time you boot them up. Don't automatically assume your computer is frozen. One time when starting up a WWII scenario I had to wait close to an hour, while the screen said that it was configuring the scenario. Just be patient, and maybe bring a book.
If you want to uninstall the mod/scenario, just go back to the scenario file and delete the biq file and game files that contained the custom info.
And for custom units....
Scroll down to the civIII section, and you'll see a subsection called creation and customization. There is yet another subsection within for new units that have been created by members there.
Find ones you like, then download them. What you do with the files depends on what you want to do with the unit. If you want to repace the default normal game unit with a new one (eg, replace the normal tank with a custom one in normal games) then you need to go into the game files and find the unit graphics folder. This can be found at
(place you told the comp to install civIII)/art/units
Find the tank graphics, and note the files located within. Rename the original tank art folder "Old tank" or something along those lines. DO NOT DELETE IT, YOU NEED THIS FOLDER IN CASE THE NEW ONE DOES NOT WORK OR YOU DO NOT LIKE IT. Now take the new tank file, and rename it "Tank". Rename the .ini file (the Notepad one) the exact same as the Old Tank's .ini file was named. You don't need to rename the other files inside the folder, because the ini file is what tells the comp what files to use for different animations, and the new ini file should (if the unit creator did his job well) already point to the new Tank files.
And presto, when you fire up a new game, the new tank graphics will replace the old silly looking ones (whoever built tanks that looked like that, anyhow? And the Panzer's not much better).
If, on the other hand, you want to create your own scenario with custom units, you'll have to create a new .biq file (with civeditor). If you leave the map blank, you'll get a random world every game. Name the new .biq file whatever you want. Now, here's the important part. Under the Scenarios dropdown menu at the top of the editor, select Scenario Properties. You'll see on the left side a space that says "Scenario search folders". This is where you tell the computer where the custom files are going to be located. Type in the name of a folder (you haven't created it yet, don't worry, that's the next step).
Now, create a folder (we'll call it "Nuts" just to avoid confusion) in the scenarios folder of whatever exapansion of civ you're running (I don't know if you've got Conquests yet). Name this folder EXACTLY what you put into the "Scenario search folders" spot in the .biq file. Now, that being done, create an "Art" folder inside of "Nuts", and inside of "Art", create "Units". Now, you do almost exactly what I outlined in the above Tank example: create a new folder for each new unit, name the folder after the unit you want replaced in the scenario, and rename the .ini file. It's important to get the unit names correct, however. Some have tricky spelling/space issues. For example, Modern Armor may be spelt in some places in the file as "Modern Armour", or Modern Paratrooper might be ModernParatrooper. Be sure to get the names exactly right, or the new custom artwork won't appear in the scenario.
That should be it, I think. Good luck.
