The Qt VS Addin is not supported in VS 2015:

"Unfortunately, the Visual Studio add-in does not work anymore with Visual Studio 2015, as Microsoft changed their plugin infrastructure, but we are providing an updated version of the plugin for older Visual Studio versions." (http://blog.qt.io/blog/2016/03/16/qt-5-6-released/)

Like @sWin said, you have to use the QT Package from Microsoft: (https://visualstudiogallery.msdn.microsoft.com/c89ff880-8509-47a4-a262-e4fa07168408)