Try adding a link to Microsoft.VisualStudio.QualityTools.UnitTestFramework, which can be found in C: \ Program Files (x86) \ Microsoft Visual Studio 11.0 \ Common7 \ IDE \ PublicAssemblies
Then, also make sure that you are using the assemblies to which you added the link.
using Microsoft.VisualStudio.TestTools.UnitTesting;
and delete the old
using Microsoft.VisualStudio.TestPlatform.UnitTestFramework;
EDIT
You can also just upgrade your visual studio to update 3, which should solve this problem.
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