TFS 2010 Custom Workflow Activity Setup

I am trying to get TypeMock isolator working as part of a TFS 2010 build workflow template. I have read the posts on getting the Custom build activity integrated into the build template. I have been able to download the custom activity and add it (ExternalTestRunner) to the Visual Studio 2010 toolbox, however when I try and drag it onto the design surface for the build template it will not let me drop the custom activity. All I get is a circle with a slash through it indicating I cannot drop the activity. I can drop system activities on the build template just fine, but not the ExternalTestRunner. Any help would be appreciated.