Apple Remote compatible with iMac?

macOS Sierra: About the Apple Remote

macOS Sierra: If your Apple Remote isn’t working

If you want to fully control your iMac remotely and you own an Apple mobile iDevice, there are remote apps you can use on both your iMac and iDevice. These apps work over Wifi and/or Bluetooth.

Apple has its own remote app simply called Remote.

Another, which I use and is very comprehensive remote app, is called Rowmote Pro by a developer called Regular & Rhythm.

http://www.regularrateandrhythm.com/apps/rowmote-pro/

If you want complete and total remote control access to your Mac, look at Splashtop Remote.

There is also a dedicated WifI and/or Bluetooth remote for Macs and iDevices by a company called Satechi.

Found here.

http://www.amazon.com/Satechi-MediaRemote-Bluetooth-Multi-Media-Control/dp/B0082 4948U/ref=sr_1_1?ie=UTF8&qid=1391729056&sr=8-1&keywords=Satechi+remote

OR

Here's an external IR remote solution.

Mira external USB/IR remote

http://twistedmelon.com/mira/hardware.html

What year is the iMac, if it was made after 2011 no, the 2012 and later models do not have an infrared sensor

Then it should work, however you did not mention how you intend to use it. As I remember their uses were pretty limited.