Man adding AI with Sapien is so amazingly easy. You can even let marines drive hogs if you change some flags in guerilla.
And what do you mean right files? If you downloaded the HEK it should have installed all the necesarry files.
In custom edition: editing campaign maps is easy as pie again, you do need to learn the halo scripting language preferably though, but that too isn't that hard.
Heck, I'm even making my own campaign map. If you want, I can help you with halo CE related questions. But we really should take this conversation to the halomods or the gearbox forums.
I am the pain, pain in the ass.