GarageBand stores the shared files in the old "Documents in the Cloud" section of iCloud, not on icloud Drive.
You will see the iPad songs when you launch GarageBand and and use "File > Import":
See: http://help.apple.com/garageband/mac/10.0/#gbnde5194bdc
- Choose File > iCloud > Import GarageBand for iOS Song.
- In the Import GarageBand for iOS Song dialog, select the song you want to import.
- Click Import.
If you want to access the GarageBand for iCloud files in the Finder, you can reveal the folder using the Terminal, because they are hidden in the user library:
Open the Terminal.app (it is in the Applications folder in the subfolder Utilities).
Copy and paste this line into the Terminal window and hit return:
open ~/Library/Mobile\ Documents/*garageband10
This will open a Finder window with your iCloud GarageBand10 songs.
And this line
open ~/Library/Mobile\ Documents/*mobilegarageband
will show you the folder with the GarageBand projects created on your mobile devices.
Once the folders are showing drag them to the sidebar or create an alias so you can access them more easily.