It said that they were already activated but I could not see the Terminal buildings. I decided to de-activate them and activate them again.
As explained, many times already, if you don't see buildings, the issue doesn't have anything to do with activation. If you lose your activation for any reason (change of hardware, registry cleanup, Windows reinstall, etc. ) the buildings will NOT disappear! The product will REVERT to Trial mode, and Trial mode doens't mean "no buildings", it means that the WHOLE scenery will work normally, for 5 minutes at time, until the next FSX restart, when another 5 minutes Trial will start. Trial NEVER expires.
So, if you don't have any buildings, the problem is NOT related to activation in any way, but there's some problem in your system which prevent the software from running
The usual question: Do you have BOTH the "Addon Manager" AND the "Couatl powered products" menu under the FSX "Addons" menu ?