The “Choose Folder” button does not work in GarageBand.
For some reason, the plugin’s “Choose Folder” button doesn’t work within GarageBand on some Macs.
To fix this, do the following:
1. Rename the instrument folder ending in “Instruments” to “.instruments”
For example, if the folder is called “Sapphire Guitar Instruments”, rename it to “Sapphire Guitar.instruments”
2. Move that instrument folder to Library/Audio/Plug-Ins/Components
3. Done, now GarageBand will find the folder automatically.