Things to consider:
1. This is a port of Nunit to Silverlight. That means you won't be able to use the Nunit runner.
2. To run the test, you run it via f5 after making the test page the default.
http://greenicicleblog.com/2010/04/27/nunit-for-silverlight-updated/
Answer from stackoverflow
It's a lot easier if you can upgrade to standard/pro and install ReSharper - it's got a really nice integrated test runner. However, if you can't do that...
* Launch the NUnit GUI
* Load the relevant tests, from debug builds
* In Visual Studio, attach to the NUnit GUI process
* Set your breakpoints
* Launch the relevant testIt's a bit of a pain, but when you've got the shortcut keys memorised (Alt-P is attach to process, IIRC - but check) it's not too bad