I've had Garageband 11 installed since I got my MacBook, but have never had the default Effects (like Bass Amp, Overdrive, and Phaser). I re-installed the app from the App Store, but this was the only thing that fixed it for me. If this doesn't work for you, I'm afraid I can't be of much help, but I hope this fixes it for you.
For starters, this article pointed me in the right direction.
- First, check to see if you have folders in this directory: /Library/Audio/Plug-Ins/Components I had many folders.
- Second, copy all of these folders to this directory (I overwrote the few that were in there, but you may want to back up what you have): ~/Library/Application Support/GarageBand/Instrument Library/Plug-In Settings
If you want to do all of this from the command line, this command should do the trick:
sudo cp -R /Library/Audio/Plug-Ins/Components/* ~/Library/Application\ Support/GarageBand/Instrument\ Library/Plug-In\ Settings