To run the GarageBand '09 'Learn to Play' lessons on a PowerPC Mac, go to the following folder: /Library/Application Support/GarageBand/Learn to Play/Basic Lessons.
[robg adds: I haven't tested this one; for those who don't know, GarageBand '09 requirements include an Intel-powered Mac for using the Learn to Play lessons.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090205212636981