im not a genious at this, but Ill tell you what worked for me.
First off, you must have the original game (not platinum hits, and some say that the online enabled vs does not work either)
1. get ltools MA vs 1.75 game save exploit from usual places.
Heres a hint: operating systems/linux/distributions. (its in there somewhere)
2. unrar it with winrar (if you dont have it, get it from download.com
3. Put it on your mem card via action replay.
4. Put mem card in controller, boot up xbox, go to memory, highlight mem card, higlight Mech Assault, arrow to the right once, push A, select copy to hdd, reboot with MA game in tray and mem card out of the controller, go to load game and select run linux, follow the intructions from this point on, reboot after linux has generated a backup of your hdd.
also, know that I never got MA working, i kept getting "cannot load run linux"
So instead, I used 007 AUF, and had no problems what so ever. So keep this in mind if you never get MA working.
good luck