In order for tests to use Typemock, they have to run in a context. In your case, the best way to go is to use TMockRunner. It is an executable, residing in the Typemock installation directory. In the simple case it doesn't take any parameters, other than, in your case, the NUnit path and parameters.
Here's a link to CCNet plug-in Gmoorick has developed:
Without the plug-in, you can do this: In the NUnit part of the CCNet file, replace the path with the following (using TMockRunner real installation location):
<path> C:\Program Files\TypeMock\TypeMock.Net\TMockRunner.exe C:\Program Files\NUnit 2.4.3\bin\nunit-console.exe
Then NUnit will run in the context of TypeMock.
Let me know if you need more help.