Your profile signature says "GarageBand '11" and Mavericks. Have you already installed GarageBand 10.0 as well? It comes free with Mavericks. You can use it in addition to GarageBand '11. In GarageBand 10.0 you will have drummers and arpeggiators. And as an owner of GarageBand '11 you can download the in-app purchase for free: Expanding GarageBand for Mac (v10.0) content with an in-app purchase
Also, if you buy Mainstage 3, you can add the loops from the JamPacks to the Loop Library in GarageBand.