For a write up - check the Simviation Design Utilities Page (Check link in my sig)
Bascially, the current versions are:
gmax 1.2 - downloadable from the discreet web site -
http://www.discreeet.com, or, if you have FS9/ACOF, it's in the extras folder of disc 1.
Caution: *at this time* - unless you have FS2002Pro, you cannot convert a gmax model into a flying mdl file.
FSDS 2.24 - commercial program - download/purchase from Abacus Publishing:
http://www.abacuspub.com