You mean you are missing the entire "Addons" menu ? Do you have other addons that are supposed that menu, or it's just GSX ? Do you have it "before" and it disappeared, or you never had it ?
Windows 7 x64
I'm afraid this might be a problem: as you can see from the system requirements, Windows 8.1 or higher is supported, since after Windows 7 came entirely out of support last year, we removed from the supported OS, since we cannot guaranteed anything will work on it after Microsoft stop support ( any updates of the C++ runtime libraries we need to compile the software might break it ).
So, while I cannot say for sure the software isn't running because of Windows 7, it's impossible for us to verify that, which is why we only list 8.1 as a minimum requirement. Of course, since all our products are sold in TRIAL version, you can try to install it to see if it works on an unsupported OS, but if it doesn't, we cannot help you much there, since nobody is testing Windows 7 anymore right now, considering it has been more a whole year since it went completely unsupported.
