What you are searching for is nunit-silverlight. You need to reference "NUnit.Silverlight.Framework.dll" and "NUnit.Silverlight.Compatibility.dll" (SilverLight 3 versions), and make all references "local copy", even the .NET framework.
If you use Visual Studio Express Edition, there is still some obstacles that I detailed a bit more on my blog. In short: Express Edition doesn't include tests or add-ins, and it should work fine with external test runners but for my part I've got an exception in NUnit's GUI, so one may need something else to run his tests (NUnit-Console, Gallio, Autotest.NET, ...). I use TddBarForPhone (available on codeplex), a tool I made which runs NUnit-Console after each build and color the results in red or green.