Stuck on Just Deserts. The guy doesn't have a beer symbol above his head..

The beer is available from the floor above, not the three cups that go on the bar.

Got this from another forum. Switch to a char. With a yellow spell (mine was Harry) and blast the clouds with yellow spell, then switch back to the char. With the beer and deliver.

I've just solved this problem! The Focus spell (the yellow spell) that you need to shoot the cloud over the head of the professor so that you can hand him his drink, is obtained in the initial story mode. If you don't have it yet, then you need to start the game all over again right from the beginning. As you play the games in the stories, keep following the ghost, who will lead you to games and to the classes where you are taught the spells which then become available to you (these classes are done in between the games, but can't be accessed anymore afterwards!). Right after the game where you become friends with Grawp, the ghost will lead you to Snape's lesson for the focus spell. Go through the lesson and then the spell will be added to your spell wheel. You can then proceed and do that game. Good luck
Lisa