I'm not 100% sure about doing the softmod thingy but here's a couple of things you can try to get it working:
- Your flash drive must first be formatted to FAT32 before plugging into XBOX (15604 blocks). If you done that it should be detected in the xbox.
- The Mech Assault hack only works from what I know with the version: Part No. X08-82262. Look for this part number on the actual disc artwork.
- From what I can see your flash drive got more than 8000 blocks but less than 16000 blocks so you have to put the MechInstaller-1.0-FATX-256MB.img onto the drive. Use a program called OzXMemory Stick Explorer. Get that at:
http://xbox-linux.sourceforge.net/down/ozxmemory.exe. It can write the MechInstaller FATX images files to your memory stick.
- Connect the USB stick to your X-box goto memory then controller the x-box will erase and format the stick for you
- Connect the USB stick to your PC and launch the program and load the 128MB file.
- If you then plug the flash drive into your xbox it should detect it..

Good luck