How do you give promotions based on a certain trait in BTS?
ex: giving combatI to melee for aggressive, or drillI to archers for protective.

Take a look at Assets/XML/Civilizations/TraitInfos.xml. This can be found in your Civ4 & Warlords directories, or BTS if you never got Warlords. You can see the examples of Aggressive and Protective in the BTS/Warlords TraitInfos file. Remember that any promotion must be given to all the unit types, so you can't give just archery units March while giving just gunpowder Str I.

I cant find Traitinfos in Assets/XML/Civilizations. I do not have warlords installed, just Civ and the BTS expansion.

Are you saying that BTS relies on the original Civ file? If so, how do mods make use of it?

Since you never got Warlords, you should be able to find it in your Bts\Civilizations directory. Info about the contents of the file itself may be found here
http://civ4.wikidot.com/xml:civ4traitinfos

Still couldn't find Traitinos.xml. I ended up copying the one from the original and fixed it up, according to the bts schema. It's being read fine and I'm no longer having problems. Thanks for your help.