I need to click the toolbar icon twice in order to make the window appear again
If you had to click twice, it's because you were using it incorrectly, closing it with the toolbar instead of using the hotkey or the x icon, which just hides the menu, which is how it's supposed to be when GSX is in use.
Quoting from Page 86 of the manual, where the usage of the menu is explained:
This is now your normal workflow should be:
• Open the GSX menu from the Toolbar icon when first starting Ground services.
• Don’t close it from the Toolbar while Ground services are performed, always use the Hot-key or the X icon to close the menu.
• Close the menu using the Toolbar icon when you are done with Ground services, usually before take-off.
it. However the installer takes forever to install. I let it run for about an hour and it looks like it hangs at "EXTRACTING Operators Thumbnails .ZIP archive in C:\Program Files (x86)\Addon Manager\texture\thumbnails".
How much free space you have on the C:\ drive ? Even if you install FSDT into a different drive, you still need about 15-20GB of free space on the C: drive during the installation ( the space will be reclaimed after installing )
If you have plenty of space, that might be caused by antivirus interference so, be sure the whole "Addon manager" folder where you installed FSDT into, is added to the antivirus EXCLUSIONS.
If still doesn't work, perhaps you got a bad download so, removing this folder:
%APPDATA%\Virtuali\PackageCache
Will trigger a new download.