Having the same problem here. I was kept referring to the Companions Quest in the CK to see how they did it, and I'm pretty sure the only reason they're able to get away with simply using Game.GetPlayer().AddToFaction(CompanionFaction) is because they include it as a property in their script(s) for the quest. I did the same thing, but I'm still getting the exact same fail when I try to compile. So I'm really stumped here as well.