To start any mission do the following:

1. Go to the option screen and highlight PASSWORD 
and press Enter.

2. In the space next to PASSWORD type ZZZCYX.

3. A new menu item will appear on top of all of the 
other options called LEVEL SELECT: to allow you to 
select 9 different levels to start from.

4. Highlight this new option and use the left and right 
arrow keys to select a level.

5. Highlight BACK TO MAIN and start the game.