Missing plugin "GB Synth"

I installed Garageband for iPad and discovered it creates a very cool Theremin-like sound with the "Fifties SciFi" keyboard. I exported a sample song with a Fifties SciFi track into iTunes and then into Garageband 11, which I saved. The track sounds pretty close in Garageband 11 to how it sounds on the iPad. However, when I open the saved Garageband 11 file in Logic Express, I get an error message about a missing GB Synth plugin -- which I guess is what creates the Theremin sound. Is there a way to get that plugin and/or get Logic Express to make use of it?