I have some tests that use the built in Microsoft.VisualStudio.TestTools.UnitTesting, but can not get them to run.
I am using visual studio 2012 ultimat
This sometimes works.
Check that the processor architecture under Test menu matches the one you use to build the solution.
Test -> Test Settings -> Default Processor Architecture -> x86 / x64
As mentioned in other posts, make sure you have the Test Explorer window open. Test -> Windows -> Test Explorer
Then rebuilding the project with the tests should make the tests appear in Test Explorer.
Edit: As Ourjamie pointed out below, doing a clean build may also help. In addition to that, here is one more thing I encountered:
The "Build" checkbox was unticked in Configuration Manager for a new test project I had created under the solution.
Go to Build -> Configuration Manager. Make sure your test project has build checkbox checked for all solution configurations and solution platforms.