I don't believe you can put your rMBP into display target mode. I think that is reserved only for iMacs with thunderbolts, or 27 inch iMacs with display port....
Ya. A lot of people have wanted to do this very thing with an iMac....to have a second screen for their iMac when they work at home....better than just leaving the macbook in the backpack! But it looks like Apple just doesn't want to let us do it!