That's the first time I heard about this message: are you sure it's coming for KDFW ? The Addon Manager handles many products, it might be there's a problem with another scenery instead, like a serial number wrong.
If you are sure it's coming from KDFW, you can try to register it again, by cleaning the Serial Number, this way:
- open REGEDIT
- go to HKEY_CURRENT_USER\Software\fsdreamteam\Dallas
- remove your Serial Number
- Close REGEDIT, launch FSX and select "Register Serial" from the Addon Manager, insert your Serial Number again and let it activate.
