The fsx gold pack came as a box with 3 dvd's in. If you forget to install the acceleration part on the 3d dvd (acceleration dvd) you won't see the dx10 option yet.
"Acceleration" ("the acceleration update") is required for many addons to work.
(btw, for people who didn't buy accel at the time there is the free service pack 1 and 2. The service packs get the addons working as well, but accell gives you several extra aircraft. The people using the service packs don't get those.
(if I remember correctly mrzippy has fsx-standard WITH acceleration).

Anywoozels, install accell. Good thing to do anyway, as fsx will run much better on the pc after that. Plus your addon probably requires Acceleration and after that is installed it will probably display properly.