The question in subject is from FlightGear FAQ and the answer suggests to look for http://chiangt.virtualave.net/BGL/bgl_index.htmlBut that page no longer exists.Any help, pls?

in general, you simply don't.More specifically: you cannot directly import MSFS scenery into FG, however what you can often do, is importing/converting the relevant 3D models over to FG and place them in arbitrary locations.You should be able to find some pointers if you search the FG mailing list archives (i.e. search for BGL).