I just generally use a icon for units that would be obsolete or not available at the time the new unit is available. It doesnt make the build menu look nice but its functional although I often have the icons for settlers and workers disappear completely when I do it. Again doesnt make it look pretty but its functional and thats what I really care about.
Generally all you have to do is extract the unit into the art/units dir. Make sure its ini and the directory name it was extracted too are the same. ie "Modern Frigate" directory and "Modern Frigate.ini" in the directory. Use the hacked Civ editor available here. Click add and when prompted type the name of the directory ie "Modern Frigate", then just click whats appropriate.
The two most common problems I have seen with unit ad ons is getting the proper balance for a unit take alot of work. Most people dont suggest what you should use.
The other one is alot of people use sounds from other units. This is not a bad thing however the way they do it is. They suggest that you copy the files to the units directory. Instead you can edit the .ini file and point to them with a statement very easily.
To use the warrior death sound you might type ../Warror/WarriorDeath.snd (not the right file name but you get the idea on specifying path. Otherwise the additional units are a great add on to the game. Some of them are just graphical, some are new units completely and it really adds to the game. I grabbed a couple of alternate fighter graphics from another site. Now the airforces are a bit different. I grabbed the attack chopper from here and the hind from another site, Some of the races get the chopper, some get the hind. The real trick is balancing them with game. This can take several games to get right which sucks with something like the attack chopper as I give it later in the game and it takes forever to get to the point where you can test it.
