Nope, Trial mode (after 5-6 minutes) will act differently: every building will disappear, leaving only the ground and jetways so no, it's no an activation problem.
Check you are using the most current version of the scenery, if not sure, download it again.
Also be sure of the following:
- No "Compatibility" mode should be enabled for FSX.EXE, if you use Vista or Windows 7. Right-click on FSX.EXE, select "Properties", select the "Compatibility" tab and be sure *everything* in that page is turned OFF
- Be sure you don't use "Transparency Antialiasing" ( also known as "AAA Adaptive Antialiasing" on ATI cards ) enabled. This is suggested on the KDFW manual too.