EDIT: How in blazes do you edit unit art?
You'll have to be a bit more specific than that.
I'd guess, what you're asking, is how to use these units ou've downloaded. Look in Civ4Artdefines.xml (it's under assets/xml/art)
That, as the name suggests, defines the model, animations, and button image, for every unit in the game. Your simplest option is just to pick a unit that already exists, copy/paste it as many times as necessary, and fill in the fields with your own data.
The Nif field is a file path. You put the unit model somewhere, and then put it's filepath there.
the kfm field defines the animation, and is also a file path. 99% of units don't have their own animations, and will use one of the vast library of firaxis animations. Usually, the artist will specify which, if it's not imediately obvious. For things like axemen, archers, etc. Your best bet is to copy an entry entirely for a unit of that type which already exists, and just change the nif.
There's also the button. Again, self explanatory. Points to a .dds texture to use as the unit's button. You can probably get by with reusing default buttons for now.
you also need to name each artdefine you make. It is STRONGLY recommended that you follow the existing protocol. It goes like thus
ART_DEF_UNIT_<unit type>_<civ>
in this case, for an archer unit, you would name it ART_DEF_UNIT_ARCHER_FRAELIM
Always capitalise every letter when defining a constant. This is a widely recognised standard, though I'm not sure if it's actually required, but it's still a bad idea not to do.
For your warrior, name it ART_DEF_UNIT_WARRIOR_FRAELIM
and you get the idea.
For unique units, you can replace the last two with just the name of the unit. Like the Calabim vamlpire unit use ART_DEF_UNIT_VAMPIRE . This is generally to allow for the possibility of multiple civs using the same UU. which can and does happen.
Also, there are naming errors on kael's part in there, where he's specified KHAZAD instead of DWARVEN for units that aren't restricted to the khazad civ. try not to copy these things.