Hi-
As you've probably figured out (since you have it working in Logic), you use Octavox by pulling it up in an "insert", from the menu that appears when you click in an insert slot. See the attached picture for where this is in GarageBand.
If it's not appearing int the insert menu in GarageBand, try the following steps:
1. Quite Garageband
2. Make sure Finder is the active application.
3. While holding the Option key, click on the Go menu at the top of your screen and select Library.
4. Delete the following files from your User Library, if they are present:
~/Library/Caches/com.apple.audiounits.cache
~/Library/Caches/AudioUnitCache/com.apple.audiounits.cache
~/Library/Caches/AudioUnitCache/com.apple.audiounits.sandboxed.cache
~/Library/Preferencescom.apple.audio.InfoHelper.plist
6. Relaunch Garageband. It should re-scan for your plug-ins.
