GarageBand on the iPad is a completely different application from GarageBand on your iMac. It will not run on your iMac -sorry. One version is written for the IOS operating system, the other one for Mac OS X on Intel computers. Your IOS GarageBand version will run on IOS devices, like iPad, iPhone, iPod Touch, but not on an Intel Computer like your iMac. For your iMac you will have to buy the iLife GarageBand Application. That will be a much more advanced version of the program - the only drawback will be that there are no smart instruments in the iLife version od GarageBand.