(By mounted he means, you need to load the game at least once in order to verify the files.Not that you have to get on top of it and ride it at all.Although I'm sure the game would enjoy that...mmm... late night semi-helpful spam. <3)
Oh, *blush* It could also be that you need to verify, so I wasn't completely wrong and now you have two possible solutions (unless it's the same thing, in which case I'm just retarded and you should ban me from forum posts)