Using Magic Keyboard wirelessly on older OS X

My friend recently bought a new iMac, but he needed a number pad, so we exchanged our keyboards. I had a wired apple keyboard, now I have the new Magic Keyboard.

The problem is, I'm running Mavericks right now, and the keyboard doesn't connect wirelessly. It does connect if I keep the lightning cable connected. Is there any way to connect the keyboard wirelessly? Please keep in mind that I don't want to upgrade to El Capitan, because it slows down my computer.

Thanks.

(iMac 2007 20" 2.4 with 3GB ram)