This seems to be an error with the JetBrains Resharper 9.1 Ultimate Test Runner referencing an older version of NUnit. I am using NUnit 3.0 in my assembly.
[Window Title]
Unit Test Runner
[Main Instruction]
Unit Test Runner failed to run tests
[Expanded Information]
at NUnit.Core.Builders.TestAssemblyBuilder.GetCandidateFixtureTypes(Assembly assembly, String ns)
at NUnit.Core.Builders.TestAssemblyBuilder.GetFixtures(Assembly assembly, String ns)
at NUnit.Core.Builders.TestAssemblyBuilder.Build(String assemblyName, Boolean autoSuites)
at NUnit.Core.Builders.TestAssemblyBuilder.Build(String assemblyName, String testName, Boolean autoSuites)
at NUnit.Core.TestSuiteBuilder.Build(TestPackage package)
at NUnit.Core.SimpleTestRunner.Load(TestPackage package)
at NUnit.Core.ProxyTestRunner.Load(TestPackage package)
at NUnit.Core.RemoteTestRunner.Load(TestPackage package)
at JetBrains.ReSharper.UnitTestRunner.nUnit.BuiltInNUnitRunner.RunTests(IRemoteTaskServer server, Dictionary`2 tests, Dictionary`2 fixtures, List`1 explicitly, Boolean useAddins, List`1 assemblies)
EDIT: Problem goes away with upgrade to version 10 of Resharper.
As mentioned in the accepted answer, ReSharper 9 does not support NUnit 3. The solution as stated does work (i.e. update to ReSharper 10), however, for those who do not have this option (e.g. licensing), you can downgrade your version of NUnit by following the below steps:
Uninstall-Package NUnit
followed by
Install-Package NUnit -Version 2.6.4
Your tests should now run using ReSharper 9.