you're right - the link for the Nunit Extension is incorrect. I've logged this and it will be rectified. However, you don't need that link in order to add a reference to Nunit.Core. I assume you already have the following using statement in your test:
It's part of the default Nunit test template.
Now you have to add Nunit.Core to your project references. If you have Nunit installed (which you should if you want to run Nunit tests) then this DLL is located in the installation folder for Nunit. On my machine I found it in the following location:
c:\Program Files(x86)\NUnit 202.5.10\bin\net-2.0\lib\nunit.core.dll
Just add it to your project references from this location.
the Telerik team