The 612 needs a SW update before it will work with the sling guide. It does not have the "web activation" menu option under "broadband setup" in the Installation menu as the 622/722 do. Once again us 612 users are on the outside looking in. I am convinced that there must not be very many 612's in the field since Dish seems very lax on fixing the many ongoing problems that many of us are having with this box. "Soon" is the only answer I am able to get from tech support on when a fix is coming.
I'm guessing something has changed since this thread - but just as FYI for anyone else searching through the forums... I just successfully added all 3 of my receivers on dish.sling.com. I have one ViP722 and two ViP612's. (So the ViP612's will in fact work). All of mine seem to work great.