That's good news.
You now need to work out which softmod you have. The easiest way is to contact the seller and ask them.
If that doesn't work, start the Xbox with 007: AUF in the DVD drive. When you get to the load game screen DO NOT select load game but start a New one. Then when you reached the actual game (after the cutscene with the helicopter), hit the Start Button and select Quit Game. Then go back to the Main Menu, Load Game screen and this time select "Load Game". Load the gamesave, and it should give you the softmod install screen.
Don't select any options, just post back with a list of the menu items you see, any names on the background, and any screensaver text that appears.