Then you definitely know how to switch windows(3D View ---> Buttons).
A little guide on how to use the G3D tools installed in the Blender download I posted earlier:
To import a G3D:
Select the scripts option from the switching windows menu.
Open the scripts menu and go to: Wizards --> G3D Fileformat Im/Exporter.
Click load G3D and select the G3D you want to load.
Switch to the 3D View window, and that's it.
To export to G3D:
Click/Select the File Menu, go to: Export --> G3D XML Exporter, save it and you're done.