Sounds like you don't have the alignments.lsx added to your mod. The editor is still very much work in progress and we're having to use a few fan-made tools to get things working like we want. This is one of those cases, sadly.
First, download 'LSTools',
http://www.divinityengine.net/index.php?title=List_of_utilitiesBefore we start, make a new folder somewhere convenient, desktop for now would be ideal, call it anything you like, 'Unpacked files' or whatever. Now, install LSTools somewhere convenient, open it, go to Tools->Unpack. You'll be asked to first select a .pak file to uncompress. You'll find the ones you want in \Steam\SteamApps\common\Divinity - Original Sin\Data (or the appropriate main install location if you got the game from GOG or Vault). There'll be several .paks in there, I'm not actually 100% certain which contains the specific file we're looking for, so possibly we'll open a few, maybe not. You want to select Main.pak, after which you'll be asked where to unpack it to. That's the new folder we made earlier. Press OK and you'll get a bunch of game folders and files. (You can do the same for engine.pak and game.pak but I don't think it's required. Again, I can't remember which contains the file we're after, but I *think* it's in main.pak.) Don't bother unpacking numbered .paks by the way, main_1.pak for example. Not necessary.
Next step is to find the alignments file and put it in the appropriate place in your mod. Go to your new folder and find \YOURNEWFOLDER\Mods\Main\Story. In here will be a folder called 'Alignments', inside of which is Alignment.lsx. This is the file you want! But it's gotta be in the right folder, so in your mod, create any intermediary folders that you're missing and then copy/paste the .lsx into your properly nested Alignments folder. The folderpathpath you create for the file will be something like \Divinity - Original Sin\Data\Mods\YOURMODNAME\Story\Alignments. Inside here will be the .lsx file.
You're now set up to use combat in your mod. All you need to do is apply the correct AI types in the relevant sidebar section.
I think they're something like
HeroPlayer1
Companion1
Good NPC
Neutral NPC
Evil NPC
Animal
but don't quote me on that.
If you have Notepad++, feel free to open up that Alignments.lsx and take a look.
Hope I didn't get any of this wrong!