[Newbies] Game Tut

[Chuck Smith]

http://squeak.preeminent.org/tut2007/html/index.html

I just started with the game tutorial and was able to figure out how to 
add the Laser-Game-Model etc to the system browser but how do I add the 
subclasses such as BlankCell, Grid, MirrorCell, TargetCell?

Any help on how to use the System Browser would be great.

Thanks,

Chuck Smith