will add NUnit to Visual Studio as an External Tools
Main Menu: Tools > External Tools
Click button 'Add':
Title: NUnit Test GUI
Command: C:\Program Files\NUnit\bin\net-2.0\nunit.exe
Arguments: $(TargetName)$(TargetExt)
Initial Directory: $(ProjectDir)\bin\Debug
Click button 'OK'.
Now you can create your assembly and test it with NUnit tool plugged into your VS.