When you load an FSX add-on aircraft for the first time, FSX should ask you if you trust the dlls (gauge files) when the airplane is first loaded. This should only happen the first time, or after a patch is applied to the add-on aircraft.
Occasionally, FSX will forget to ask you to authorize the dlls, and this can create an undesirable circumstances where the dlls (gauge files) are not loaded with the rest of the airplane. The end result is an unusable simulation.
Correcting the Problem:
The first step to correcting this problem is to open the FSX.CFG file and delete all references to your product that can be found under the [TRUSTED] section of the FSX.CFG.
Next, you must eliminate a troublesome registry setting that Windows uses for third party DLL trust policy selection.
Check if the HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\WinTrust\Trust Providers\Software Publishing registry key is set to 0x63c00.
This value is Microsoft's WinTrust policy selection flags, as described here. The 0x40000 flag means "Allow only items in personal trust database" and apparently, this clashes with FSX's ability to allow manual authorization of unrecognized DLLs, telling it instead to never trust anything outside the user's personal trust.
Unsetting this flag (putting it back to its default 0x23c00 value) returns all operations to normal and Flight Simulator X will once again be able to ask the user to designate the PMDG DLL modules as 'Trusted' software.
(Thanks to Steve Patrick at Spat's Weblog for this)