NUnit assembly not found

I had the same problem, and I had installed using NUnit-2.4.8-net-2.0.msi. Expanding on the "add to the GAC" comment, here's what I did:

• Open your "Visual Studio command prompt (generally: make sure gacutil is in your path) and: cd "C:\Program Files\NUnit 2.4.8\bin"

• Unregister your NUnit entries from the GAC. You can do this by finding the NUnit entries registered in the GAC:

  gacutil /l | find /i "nunit" > temp.bat && notepad temp.bat
  

• Prepend the nunit.core and nunit.framework lines with "gacutil /uf", i.e.:

  gacutil /uf nunit.core,Version=2.4.2.0,Culture=neutral,PublicKeyToken=96d09a1eb7f44a77
  
  gacutil /uf  nunit.framework,Version=2.4.2.0,Culture=neutral,PublicKeyToken=96d09a1eb7f44a77
  

• Run your .bat file to remove them: temp.bat

• Register the NUnit DLL files you need:

  gacutil /i nunit.core.dll
  
  gacutil /i nunit.framework.dll
  

Make sure you have added a reference to nunit.framework. If you have, then make sure the properties of that reference have the copy local property set to true.