Trackpad not discoverable

I just bought a Trackpad at the Apple Store.

I have an MacBook Pro 13 inch, mid 2009 with OS X Lion and an iMac 21.5" with OS X Yosemite with the latest update.

The Trackpad works fine with the MacBook Pro.

I have been trying to pair it with the iMac and it just does not pair. The iMac continues searching but the Trackpad itself does not blink as expected in Discoverable mode. I then tried to disconnect the TrackPad from the MacBook Pro because I thought it may only pair with one computer. So far have been unable to find how to do it.

If I can use on both it would be better.

If not, how do I disconnect Trackpad from the MacBook Pro and what am I missing to pair with the iMac?

Please help!