Okay, two questions:
First: Did you install all updates on the Mini? There were some initial problems with Thunderbolt.
Second (and this may be stupid, but just in case): You are absolutely sure you got the 2011 Mini with TB, not the previous model?
If everything is up to date (even Mountain Lion), just try restarting while the TBD is connected. If this doesn't help, you'll have to call Apple.