All you have to do is to add references to the nunit.core and nunit.core.interfaces dll files from your version of NUnit. You can find them in C:\Program Files (x86)\NUnit 2.5.7\bin\net-2.0\lib
After that you have to copy the ArtOfTest.WebAii.NUnitExtension.dll from the project's bin\Debug folder and paste it in the C:\Program Files (x86)\NUnit 2.5.7\bin\net-2.0\addins folder.
I have recorded a short video as demonstration of the process. You can also find attached the ArtOfTest.WebAii.NUnitExtension.dll that should work with NUnit 2.5.7.
Looking forward to hearing from you.