Unlock bonus mission:
     Start  the  game,  then at the Main Menu, select "Single Mission"
option. Then choose any Character & Name, then type the unlock code on
your keyboard. Note: The codes are case sensitive.

     You  can  unlock  the  bonus  mission  on  the  Mission Selection
sub-screen  of  either  "New  Game"  or "Load Game" also, which is the
Campaign  Mode  where  you  play  through  the  missions in order. The
"Single  Mission"  option  seems to be the exact same missions, but it
allows  you  to  play  them  in any order. The "Single Mission" option
gives access to all the levels.