How do you normally do this? This is how ATV is designed to work...not the remote speaker side of things...which is possible too certainly, but using your ATV to play all the content on your iTunes library on your Mac. The remote speaker option is awkward, since you have to be in front of your iMac to use it...the whole point is to use your AppleTV to listen to your music library.
I assume it's already linked to your iMac/iTunes Library...which is what happens when you first set it up....entering the 5-digit code and all.
Go to your ATV in iTunes (same location where you find your iPhod or iPhone on the list on the left) click it, go to the Music tab and unclick where it says to sync certain playlists to your AppleTV. ALL your iTunes music will be available...streaming from your iMac to your AppleTV and not copied to your ATV. This is the way the ATV was designed to work. You can sync some stuff, but it will mirror and stream all your iTunes content.