This can be caused by several things:
1) The window transparency feature in GibbsCAM is based upon capabilities not found in Windows 98, ME or NT. Window transparency is only available in Windows 2000 and Windows XP.
2) Transparent dialogs is not compatible with OpenGL. If you are running GibbsCAM v8 or later you will need to disable OpenGL (in Preferences) in order to use this functionality.