You cannot without paying again, afaik.

Do you have a TimeMachine backup of your system? Then try to restore the following folder in your System Library from your backup:

For example, to restore sting's Roxanne:

/Library/Application Support/GarageBand/Learn to Play/Artist Lessons/Sting_Roxanne.mwand

I just discovered, you should be able to  download the artist lessons again from the App Store, see this Support Article:

GarageBand: Re-downloading Learn to Play Artist Lessons

http://support.apple.com/kb/HT5226

Regards

Léonie

have you had any luck. The support artoicle didnt help me at all.  Once again I'll have to call Tech Support... ARGHHHH!!!

still no solution.... there's another thread i just commented on. Still no apple acknowledgement and support phone calls did nothing.