There are a number of ways of doing this but one of the simplest is to go to the NUnit site, click on "Documentation" then "Current Release" then "Features" then "Visual Studio Support" e.g. something like
Add a custom tool entry specifying the path as described and then run nunit-gui.exe by clicking the NUnit entry which will now be under the "Tools" menu.
If you want to debug using NUnit, the procedure is also described here. Essentially, you first set the breakpoint, run an instance of NUnit as described above and then you click "Tools / Attach to Process" and then select the nunit-gui.exe line in the dropdown. Then re-run NUnit by clicking the "Run" button.