Generally it is because ADE isn't pointing to the correct folders for the Sims. Go to the FSX folder inside ADE and delete a file called Settings.ini. Now start ADE and select FSX. When the new user wizard appears complete fully. On the folders tab use the Find button so the program will search for the correct paths. When you have completed the new user wizard restart ADE.
If this does not work then you may have problems with registry entries.
Repeat this for FS9. Look for the FS9 folder inside the ADE main folder and delete the settings.ini file there.