I moved your post, because you posted it in the wrong section (General) instead of the KDFW support section.
It's of course entirely normal the scenery won't ask for a serial number, because all our products for FSX/P3D are sold in Trial version so, there wouldn't be much of a Trial if you couldn't install it without a key.
Activation can be done either in the FSDT Installer or from the Addon Manager menu in the sim. However, it's not clear that if:
- The scenery disappears after 5 minutes (per session), which is what is supposed to happen in Trial mode
- The scenery doesn't appear at all. This is not Trial mode, it's software not working.
The scenery also *requires* GSX, which of course is completely free to use there, but it's not clear from your post if you have installed it, and if yes, if it works.