Several customers have had this issue occur and the solution in all cases was to do the following:

  1. Locate the NUnit Add-Ins folder
  2. Right-click on the SpiraTestNUnitAddIn.dll and choose Properies
  3. On the dialog box that appears click the [Unblock] button
  4. Restart NUnit

It should now work. The strange this is that NUnit will list the add-in as Enabled, but it won't actually load it.