Large Black Squares Appear In Your Scenery
This is caused by an outdated terrain.dll file and is fixed by either installing FSX Acceleration Pack or SP1 and SP2 below.
Microsoft Flight Simulator X Service Packs 1 and 2
For MegaSceneryEarth (or any photoscenery) to display properly in FSX, you need to either have Acceleration pack installed or FSX Service Pack 2. These updates install the latest version of the terrain.dll engine required to display MegaSceneryEarth (or any other photoscenery) correctly. The most obvious symptom if you don't have these are large black or badly colored squares over your terrain. Please note that to install SP2 you have to install FSX Service Pack 1 first.
You can download these service packs here:
FSX Service Pack 1
FSX Service Pack 2