Do you still have the Team Foundation Build Activities tab? If not, you probably deleted it. To get it back, you can right-click on the toolbox and select "Reset Toolbox". This will get it back to the default, which should contain the lost tab.
You could also add them by right-clicking on the toolbox, then "Choose Items", then "Browse" and select C:\Program Files (x86)\Microsoft Visual Studio 10.0\Common7\IDE\PrivateAssemblies\Microsoft.TeamFoundation.Build.Workflow.dll.
