Sorry. Still very much WIP.
To choose the 'single-player' mode, walk unto the "SOLO" 'door' and then pick what level(s) you want to play.
The single-player levels available at the moment are mainly there for me to try out certain concepts. I have a vastly different single-player mode in mind, but am trying to limit myself much as possible, so that it will still be playable on a stock A500, unfortunately it seems as if though how I envision it, I might need to push the limit to an A1200 w/ 4MB, at least. But for now there are the same levels available both in multiplayer and single-player, long as you walk through the door that says: "SOLO". Currently you are able to pick what level you wish to play, in any order, but upon completing a level, you will be directed to the next in line.