Please try updating to VisualGDB 5.4 Beta 2 and then right-click in Solution Explorer on an Arduino project. The “Add->Manage Arduino Boards” will be available there.
If it doesn’t appear, please share a screenshot and we will help you locate it.
Please use the context menu in Solution Explorer (right mouse click on the project name in the right part of the Visual Studio window). You can open Solution Explorer via View->Solution Explorer.
If you still cannot find it, please attach a screenshot of the entire VS window showing your project and we will highlight it for you.