Im not sure about this because I don't have the PMDG MD-11, but, anyways I'll try to help.
When the installer tells you to pick a directory does it say to choose the main FSX folder or the aircraft's main folder? Because if it says to choose the aircraft's main folder and you chose FSX"s main folder it may have overwritten one of the crucial terrain files in the main Terrain folder, or it may of just overwritten the whole thing. Just have a look in your PMDG MD-11 folder in the Simobjects\aircraft folder and see if there are other texture files like "texture.1, texture2" etc. if thats there then you have some other problem. It could be something else that you downloaded before you tried installing the MD-11.
I hope that helps, ;)
Bye.
