Right-click on the song you want. You'll get a menu of options, which include adding to playlist, adding to crate, or adding to AutoDJ queue.
I'm trying to recall the difference between the playlist and the crate. Seems to me the songs in the crate didn't give me the option of adding to the AutoDJ queue -- however it does now, but I'm using MIXXX 1.11 alpha. Whatever the reason, I arranged things such that the crates would include everything that I would ever want to play at a particular type of gig (weddings, car shows, etc), and the playlists would be extracted from the crates, and include the songs I would play at specific locations (car cruise in Venice, car cruise in Clearwater, etc.). Not quite sure if this is how it was intended, but that's the way I use it. If you "save" a crate or a playlist, these are saved as the same filetype. So you can call up a former crate as a playlist if you wish.