Currently Being ModeratedDec 28, 2013 4:45 AM (in response to powermacler)
I had the same problem.
Found out this happens when buying the extra content is the first purchase you do after installing Mavericks.
When you go to the app store, buy any app, and then go back to garagband and try again, it will work.
It did for me.
(bought an app I wanted anyway)
In my search for a solution I found another post that said it also works when you buy a song in iTunes, but I'm not sure about that.