I can give you some help with MTW historical battles: I haven't tried RTW ones yet. It all depends on what you want to do.
If you want to modify existing battles you must edit the BDF and ADF files that are in the individual battle folders. You should be able to find them here.
C:\Program Files\Total War\Medieval - Total War\Battle\batinit\Historical Battles

Get a text editor. There are many, but I use Notetab light, cos it is free.
http://www.notetab.ch/ntl.htm

If you load up the files you should be able to edit things such as unit numbers and types. If you are unhappy with unit stats you need Gnome editor, which you should find at the download section of the org. You can load the following files with this editor if you have VI.
CRUSADER_BUILD_PROD13
CRUSADERS_UNIT_PROD11
PROJECTILESTATS

The other text files relating to the add on can be edited as well.
I can do some other things, but I'm still figuring out how to do some stuff myself so I hope this helps - you can search the org if it doesn't, there are a few guides knocking around. What exactly are you trying to do?