Those Apple products can be a bit tricky at points, so I definitely understand the confusion. Luckily enough, we have a great page that describes the process of hooking up to an Apple TV. I'll provide the link below for you.
Hope this helps,
The Slingbox Moderation Team
You need OS X 10.8 Mountain Lion on your iMac.
Then, provided your iMac is recent enough, you will have an 'Airplay' option which allows you to stream what is on your screen to your Apple TV.
Unfortunately the Slingplayer plugin appears not to work under OS X 10.8, so you'll have to wait until Sling update it before you can try the above.