I have finally got back to this, so I can answer some of my own questions.
The above steps did let me see the StyleCop activity in the Toolbox in VS2012. However, after I closed the container project described in the "How to integrate the extensions into a build template" document when I tried to drag the activity from the
toolbox to the workflow I got the "no allowed" icon (the circle with a diagonal line).
After doing some more research, I found that you the assemblies that you add to the toolbox need to be put into the GAC into order to allow you to drag them to the workflow. In order to be put into the GAC, the assemblies need to be signed.
Unfortunately, the assemblies from Community TFS Build Extensions, Code Activities\VS2012 are not signed as discussed in a different thread (https://tfsbuildextensions.codeplex.com/discussions/456855
I will create an issue and a patch to cover signing all of the assemblies in Community TFS Build Extensions, Code Activities\VS2012 except for Activities.Azure as there is an issue with that project.