Apple TV not connecting to iMac

Hi all

We've bought an iMac a couple of weeks ago and last night I tried to pair it to Apple TV. No matter what I do, when I go to "computers" on ATV, it comes up with a message "No content for this library".

I've restarted everything (iMac, iTunes, ATV and Router) and nothing changes. I've shared my whole iTunes library on iMac, so it should pick it up. On ATV, I can access all the other apps, like Netflix, NowTV etc. I can even see all my pictures. It just will not connect to my iTunes library on the iMac.

I've had the ATV for a couple of years now and it worked without fail with my Acer laptop, so I find it very strange and frustrating that it will not work with the iMac!

Please help!

Unfortunately, I'm not sure which gen the ATV is, but we bought it around 3-4 years ago and never had any issues with it.

The iMac is the 21.5", 2.7GHz, OS X Mavericks.