Bummer, sounds like you ran into a bug of some sort.
After selecting a job (Either clicking on it, or Q/W) it should highlight in white. From that point you should be able to assign the selected job by clicking on a slime. The number next to a job indicates how many times it can be used. During the first couple of levels there aren't any jobs to assign, since they were trying to show a few different things.
Oh, the other part that's not explained is that you can only assign jobs while the game is playing (ie, after hitting space)