First, from your description is not really clear what your problem is.
If the scenery is not activated it will NOT disappear! It will run in Trial mode for 5 minutes. So, if it simply doesn't show up, that's nothing related to activation instead, it's always something entirely different.
There's a very simple way to tell if there are any activation issues, just select the "About" option for that scenery. If it says "Active", then IT IS active, and you can safely exclude about activation problems in this case.
If the scenery doesn't appear at all, it might be due to one of the following reasons:
- The Addon Manager is not even running. This can be easily detected, because there's no Addon Manager menu. Typical cause for this, it's a bugged antivirus program that mistakenly identifies it as an unknown virus, and disable it.
- The Scenery hasn't been added to the Scenery Library. This is quite easy to check.
- There's another scenery in conflict for the same area that is excluding it. Solution is disable it. Some sceneries, usually freeware, don't have proper uninstallers, and they usually keep trace of it, even in the scenery library or modify parts of the default scenery, that it's difficult to identify.
- Installation didn't complete and the scenery wasn't added the Scenery Library, because your scenery.cfg had a syntax error inside with an illegal command, due to having hand-edited. This it's usually easy to spot, because the installer will issue an error message.