Every iTunes library you want to control must have Home Sharing turned on
You can pair your computer with only one remote at a time. You must remove the pairing before you can use a different remote with the computer.
OS X Mountain Lion: Pair and unpair an Apple Remote
If you have an iPod touch, iPhone, or iPad, you can use the Remote app to control your iTunes library
iTunes 11 for Mac: Use the Remote app to control your iTunes library
Hope this helps,