You're going to need the Slingplayer mobile app for iPad or iPhone, start the stream on the iPad/iPhone/iPod Touch, and then use AirPlay to send the video from the iPad/iPhone/iPod Touch to the Apple TV. As mentioned above, there is no direct route from the Apple TV to the Slingbox--there must be an iPad/iPhone/iPod Touch involved.
I hooked up my Slingbox 500 to my DirecTV receiver, and now am trying to watch my DirecTV on another TV using Apple TV, but I don't see how to watch via Slingbox? Can someone help me out? I figured that I could watch it using Apple TV since that has internet access instead of purchasing Roku or Chrome TV, was I mistaken?