Richard's problem was that he's running tests inside an executable, rather than inside a process of a runner (e.g. NUnit.exe). When you run an app, this way you get the message:
Could not load file or assembly 'TypeMock, Version=0.0.0.0, Culture=neutral, PublicKeyToken?=0000000000000000' or one of its dependencies.
Like all runners, Typemock Isolator needs to be initialized when the process loads. In order to do that, you need to run the process under Typemock's TMockRunner tool. For instance, in the directory when the app is installed, use this:
"c:\Program Files\Typemock\Typemock Isolator\TMockRunner.exe" TypeMockTest.exe
This treats the exe as a runner and initializes typemock as it loads.