Cannot add unit test assembly to VS2013 project

I wish to use the CustomAttibutes collection on the types in a loaded assembly to find those types tagged with the MS-defined TestClassAttribute. I'm using VS2013 Pro (which includes unit testing, and my tests do run), and the attribute in question is defined in Microsoft.VisualStudio.QualityTools.UnitTestFramework.dll. However I can't seem to add a reference to the assembly even though the dll is listed (as an 'extension' in the Add References dialog in VS2013. Am I missing something?