Licensing is on a per-deployment server/CPU core basis. Microsoft has licensing info up on MSDN, but it's still a little bit confusing at the moment.
I suspect that you downloaded a trial version that has expired. If you have an Ultimate MSDN subscription, then you have a free license, per the licensing page.
Release Management works with TFS2010, 2012, and 2013 -- the licensing is the same for all of them, so it's not that you're connecting it to TFS 2012.