To clear up the problem, please try the following:
- Delete your dll.xml and fsx.cfg files. You need to locate your dll.xml file associated with FSX. In XP it's at \Documents and Settings\(Your User Name)\Application Data\Microsoft\FSX and in Vista/Win7 it's at \Users\(Your User Name)\AppData\Roaming\Microsoft\FSX. Note that you will need "Show Hidden Files and Folders" checked in Windows Explorer's Folder Options in order to see these folders.
- Uninstall the PMDG 747-400X from the Control Panel. (Add/Remove Programs in XP or Programs and Features in Vista)
- Manually delete the PMDG folders that remains on your hard drive after uninstalling. By default these are at \Program Files\Microsoft Games\Microsoft Flight Simulator X\PMDG and \Program Files\Microsoft Games\Microsoft Flight Simulator X\SimObjects\Airplanes Within the Airplanes folder, remove the PMDG747-400 and PMDG747-400F folders.
- Reinstall the 747-400X and the latest update.
- Run FSX, answer yes/run to all the dll popups.
- Select the 747-400X and load it, answering yes to the gauge popups.
This should solve the issue. If it doesn't please contact support for further troubleshooting.