I\'m using Visual Studio 2013 (Update 3) with git & Visual Studio Online as Source Control.
I can add tags to work items in the Visual Studio Online web experience b
Adding and editing tags in Visual Studio 2013 has been enabled in Update 2. I'm currently running Visual Studio 2013 Update 4 and the functionality is available there.
Make sure you download and install the latest version of Visual Studio 2013 to get access to the latest goods.