Hello ThomasL.
Have you ever had the activation prompt open up, like on first run? Or has it just never worked at all? If it hasn't try the following:
Press Start -> type regedit into Start Search and press enter. Elevate when prompted.
Make your way to the following key: HKEY_LOCAL_MACHINE\SYSTEM\CURRENTCONTROLSET\CONTROL\CLASS{4D36E967-E325-11CE-BFC1-08002BE10318}
Find Upperfilters listed under it, and check its value, if its 0 change it to PartMgr and click OK out.
If that worked you should get the activation prompt the next time you run FSX.
Paul Smith - MVP for Windows Desktop Experience... I've crawled off NNTP - for now. Posting from Aldershot, UK. On the internet atwindowsresource.net and dasmirnov.net. Please post back letting us know what works and what doesn't. :-)