Unfortunately, there is only a Spec Explorer extension to Visual Studio (VS) 2010. Many questions of whether and when Spec Explorer will be available for VS 2012 have been asked in the original Spec Explorer forum. The answers varied, and the forum has now been locked and set to read only mode.
Chris Struble has asked about Spec Explorer for VS 2012 in the general forum about Test Tools in VS 2010&2012, explained the situation in his blog and started a feature request for Spec Explorer for VS 2012 . So if you care about the future of Spec Explorer or successful applications of MBT, please vote for the request!