Furon Invasion:

This cheat makes more furons destroy humans. Just look for a yellow square these are the places. Note there is not a square in fantasy atoll.

Harbor city: go to the ships with those big rectangle things.on the first ship go around or over it and look back at it. Onder a group of them there should be the yellow square walk into it to summon furons.

Fairfeild: go in the city and find a different farm. Near the farm should be a corn feild. Walk around in it until you find the square. Walk in it to summon furons.

Vietmahl: first cross the river opposite from the big willy restaurant. Then cross the only fixed bridge. There should be a army fort there go to the back of the building and theres the square. Walk into it to summon furons.

Note: you must beat the game to do it.