How do I get my new Mac USB 3 to recognize my old keyboard USB 2?

Hello Jako285,

Thanks for using the Apple Support Communities!

Being able to keep using favorite things like keyboards is always nice when getting another system.

As you can see from the information below, your device should be backwards-compatible with USB 2 ports. As long as the connector fits into the computer properly, and the keyboard is Mac compatible, it should work without any problems.

The back of the iMac (Late 2012 and later) has a total of four USB 3.0 compliant ports. You can connect compliant USB 3.0, USB 2.0, and USB 1.1 devices to these ports.

iMac external features, ports, and connectors

Cheers!