Add Animal Companion to available list
 

Is there a simple way to add a given AC to the list of allowed ACs for a character?

The easiest way is to go into the editor go-to Race then Race, Animal Companion copy the animal companion you want then go to minion allowed for and select the class and save the new animal companion and you will be able to select the new Animal companion on the class tab for your selected class. You can also make new animal companions this just takes a little more work.