In the end I gave up on configuring the MSTest through the easy panels - it would not find the test dll, so like the guy above I used the batch command. I have added my batch command below because it addresses some issues with previous answers that they do not take into account that you have to delete the .trx file before each run, or give it a unique name or tests will fail.
Here is my solution running in Jenkins as a batch command:
"C:Program Files (x86)/Microsoft Visual Studio 12.0/Common7/IDE/MSTest.exe" /resultsfile:"%WORKSPACE%/MyAppTest_%BUILD_NUMBER%.trx" /testcontainer:"%WORKSPACE%/Components/ebox2/Central/MyApp/MyAppWeb/trunk/MyAppTest/bin/Release/MyAppTest.dll" /nologo /category:Build
Note the inclusion of %BUILD_NUMBER% in the .trx file name, which gets round the problem of jobs failing because of duplicate .trx file.
Note also that /category:Build allows you to choose the tests you want. You set up the category in the tests themselves:
[TestCategory("Build"), TestMethod()]
I use this because I have some Selenium tests which I have not figured out how to run in Jenkins yet, or if this is possible.