I have a cake with a fondant ribbon on the side of the cake and a fondant bow attached. How do I attach this bow so it won't fall off?

Royal Icing... dries like a rock, and is plenty sticky enough to hold it till it does dry.

I've read that melted chocolate works, too.

I sometimes use a bit of water or flavoring to stick something to fondant, if you don't have a lot of time I would suggest the royal icing, it dries faster than using the water or flavoring. Good Luck