Also, to be clear: you can't use those files as a "reference" on how to add artwork for your new unit. These aren't GameData files; you have to, in your mod, replace the old file IN ITS ENTIRETY. That means that YOUR version of CIV5ArtDefines_unitmembers.xml or whatever must include the art definitions for all of the existing units as well. This'll make your mod incompatible with any other mod that adds units or modifies unit graphics in any way.
You don't do this through the usual OnModActivated command; this has to be handled through the VFS instead.