Several customers have had this issue occur and the solution in all cases was to do the following:
- Locate the NUnit Add-Ins folder
- Right-click on the SpiraTestNUnitAddIn.dll and choose Properies
- On the dialog box that appears click the [Unblock] button
- 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.