The same happened to me when I tried to start the game through Content Manager. I just had to launch Content Manager as administrator and it worked. I couldn't find the "steam_appid.txt" file either, so I looked it up. There was one guy on a thread unrelated to AC, and he was talking about creating the file through other game folders. I don't think you need the file, though, since I didn't need it to work for me.
If you already fixed it, let me know. Would love to know how you did it.