It has eluded most players on how to play against the computer. I'm sure it is buried somewhere in the long documentation file. The game defaults to a two player mode. Select options with the mouse to get to the options screen. Click on the word "first". That is the first level of the two player codes. Type in 'conra' or code for any other game level that you wish to play. Then select exit to play the game.
00 - "CONRA" | "FIRST" | "EUROP"
01 - "PHASE" | "GHOST" | "STORM"
02 - "EXOTY" | "GAMMA"
03 - "MOUNT" | "MARSS"
04 - "FIGHT" | "EAGLE"
05 - "RUSTY" | "METAN"
06 - "FIFTH" | "FOTON"
07 - "VESUV" | "POLAR"
08 - "MAGIC" | "TIGER"
09 - "SPACE" | "SNAKE"
10 - "VALEY" | "ZENIT"
11 - "TESTY" | "DONNN"
12 - "TERRA" | "VESTA"
13 - "SLAVE" | "OXXID"
14 - "NEVER" | "DEMON"
15 - "RIVER" | "GIANT"