The button should show up under File|SourceControl|Go Offline.
It is shown if the following three conditions are true:
1. VS supports TFS.
2. The solution has valid bindings to TFS.
3. The solution is not already offline.
However, due to the architecture of VS, these conditions are only evaluated once the extension has been loaded. The button is always visible before the extension has been loaded - ie: right after VS has started before you open any solutions.
Great extension! I was wondering if it was possible to add it to the right click menu in the solution explorer as well? It would be nice to be able to right click on the solution or project etc and click Go Offline instead of having to navigate to the File menu.
Once offline there is a 'Go Online' option in the right click menu.