From your explanation of the problem, it's not clear if it's only GSX that doesn't show up, or it's the whole software modules needed to run it.
So, the usual questions:
- Do you have BOTH the "Addon Manager" AND the "Couatl powered products" menus under the FSX "Addons" menu ?
- Do you have both of them, none of them, or just one ?
- Do you have other FSDT products installed ? Do they work ?
- Have you being ASKED permission to run the two modules when you first started FSX after installing ? Have you replied YES to BOTH questions ?