(I see jetways but no buildings, which I believe means trial mode.)
That's not Trial mode. Trial mode is: all the scenery, visible for 5 minutes at the time. No buildings, right from the start, means "software not running", so your problem doesn't have anything to do with activation
However, the add-on manager says KLAX is installed and active!
That's because it IS active. As I've said, your problem doesn't have anything to do with activation.
Do you also have the Couatl menu under the "Addons" menu ? If not, this explains why the scenery doesn't show, and the most common cause for this, is that an update of the antivirus, has introduced a bug, which is blocking the program from starting, since it has mistakenly flagged it as a threat. It isn't, of course, it's just an antivirus bug.
- Configure the antivirus to exclude the Addon Manager folder.
- Download and install the scenery using the latest installer. Be sure you keep the antivirus DISABLED when downloading and installing.