This is a sample script that you can include in your build process. Note that the use of the <BuildStep> tasks are optional and you can remove those if you desire.
<Target Name="AfterCompile">
<!-- Create a Custom Build Step -->
<BuildStep TeamFoundationServerUrl="$(TeamFoundationServerUrl)" BuildUri="$(BuildUri)" Name="NUnitTestStep" Message="Running Nunit Tests">
<Output TaskParameter="Id" PropertyName="NUnitStepId" />
</BuildStep>
<!-- Get Assemblies to test -->
<ItemGroup>
<TestAssemblies Include="$(OutDir)\**\cs-*.dll" />
</ItemGroup>
<!-- Run NUnit and check the result -->
<NUnit ContinueOnError="true" Assemblies="@(TestAssemblies)" OutputXmlFile="$(OutDir)nunit_results.xml" ToolPath="$(ProgramFiles)\Nunit 2.4.7\bin\">
<Output TaskParameter="ExitCode" PropertyName="NUnitResult" />
</NUnit>
<BuildStep Condition="'$(NUnitResult)'=='0'" TeamFoundationServerUrl="$(TeamFoundationServerUrl)" BuildUri="$(BuildUri)" Id="$(NUnitStepId)" Status="Succeeded" />
<BuildStep Condition="'$(NUnitResult)'!='0'" TeamFoundationServerUrl="$(TeamFoundationServerUrl)" BuildUri="$(BuildUri)" Id="$(NUnitStepId)" Status="Failed" />
<!-- Regardless of NUnit success/failure upload the results to TFS. -->
<Exec Command=""$(MSBuildProjectDirectory)\NUnitTFS.exe" -n "$(OutDir)nunit_results.xml" -t "$(TeamProject)" -b "$(BuildNumber)" -f "%(ConfigurationToBuild.FlavorToBuild)" -p "%(ConfigurationToBuild.PlatformToBuild)" -x "$(MSBuildProjectDirectory)\NUnitToMSTest.xslt"" />
<!-- If NUnit failed it's time to error out -->
<Error Condition="'$(NUnitResult)'!='0'" Text="Unit Tests Failed" />
</Target>