This article is for Visual Studio 2005.
Step 1. steup your [TestFixture] and [Test]. And open the dll file from NUnit-GUI.
The detail of Step 1 is shown here: http://www.tangent-studios.com/programming/csharp/NUnit2Tut/NUnitV2Tut.htm
This is a small and easy tutorial, just spend your 5 mintues.
Step 2. Add breakpoint into your NUnit [TestFixture] class in Visual Studio.net 2005.
Step 3. Click "Run" in NUnit.
Step 4. Click Debug-->Attach To Process
Select 'nunit-gui.exe" from "Available Processes" and click "Attach" in Visual Studio.net 2005.
Notes: Step 3 can not be skipped, otherwise there is no active process in "Available Processes".
If you recompile [TestFixture] class, have to redo step 4 to attach the process again.