There has been some confusion about how you access the additional content in GarageBand for iOS and Mac. Today, Apple posted two helpful articles explaining what content comes with the apps by default and the steps you need to take to download the extras. It’s worth reading both the iOS and Mac support articles.
Expanding GarageBand for Mac (v10.0) content with an in-app purchase
Expanding GarageBand for iOS 2.0 content with an in-app purchase