aah, so you probably can get help. First open the mb file with the lowest maya version possible. Then save the file as maya ascii, open it in a text editor and look at the first few lines (maya 2009 file):
//Maya ASCII 2009 scene
//Name: export.ma
//Last modified: Sun, Jan 21, 2011 12:08:12 AM
//Codeset: 1252
requires maya "2009";
requires "Mayatomr" "10.0.1.8m - 3.7.1.26 ";
requires "stereoCamera" "10.0";
currentUnit -linear centimeter -angle degree -time ntsc;
fileInfo "application" "maya";
fileInfo "product" "Maya Unlimited 2009";
fileInfo "version" "2009 x64";
now check all lines beginning with "requires" especially those pointing to plugins your maya v6 doesnt have ( here: this version of mayatomr as well as the stereo camera). Delete them. And for the remaining lines change maya version number entries to your maya 6.0. Now try to import in maya 6.0. It will give you errors because it will read many attribute extensions it doesnt know but it should open the file with some luck. If not you at least should get an more detailed message on what the error causes and you could manually fix it in your ma file.
