Remote not working on new iMac
The new iMac does not have an infrared receiver. You will not be able to use your Apple Remote. For controlling iTunes, I recommend Remote from the App Store. For Plex, I recommend Plex from the App Store.

Whereas in the past Apple promoted the idea of using your Mac as a media hub (hence the inclusion of the Front Row app), I think Apple is now promoting the idea of using an Apple TV instead. It seems like every new hardware refresh is having the infrared capabilities removed.