SPE is great, but I prefer eclipse because of code-completion and svn integration.
All the stuff related to the engine are inside M30W.scripts and M30W.sprites.
Make sure you're at rev107
The bug in the costume center - I tested it on Windows 7, and it's probably still there because I haven't changed the code since - is that you cannot edit it because the controls (wx.SpinButton or whatever it's called) don't respond to mouse clicks - that also makes it impossible to type in.
And if you need something to do you can also make a command-line interface.