It is possible to use NUnit as an external tool in the new 2010 version.
The first step is download and install the latest version of NUnit since older versions are not compatible with .NET 4.
Now go to Tools -> External Tools:
View -> Toolbars -> Customize:
Press the New button and enter a name. Now open the Commands tab and add the external command (usually External Command 1 when you don’t have any other external commands) to the toolbar:
When clicking the NUnit button in the new toolbar the NUnit GUI starts and loads your test suite.