Here are the two cheats that I know of (there may be more, but the programmers
haven't told us about them yet if there are):
 
To be able to play Military vs. Military:
 
At the main screen (with the MISSION BRIEFING, etc. on-screen), press the left
and right arrow keys sequentially, i.e., LEFT, RIGHT, LEFT, RIGHT, until the
screen flashes.  You'll be able to play Mil vs. Mil.
 
To be able to fight the Supervisor:
 
At the main screen, press two lefts then two rights alternately, i.e. LEFT,
LEFT, RIGHT, RIGHT, LEFT, LEFT, RIGHT, RIGHT, until the screen flashes.  You'll
be able to play player vs. player with player 2 as the Supervisor, or you can
go to Training and play with the computer as the Supervisor.
 
- Sean
  TWI Online Support