On San Francisco (being Venom) get to the ATM machine. When there get in front of it by the pillar. Then face to the right and do your special attack to enter the bonus round.
Defeating Carnage, Secret Room
To defeat carnage quickly, use a daggerhead or the sonic gun (although the daggerhead works more quickly). In the second stage ("The End"), when you get hit, while you're glowing, move close to Carnage and keep punching him rapidly until you gain your special, and then kick him. In the final stage ("Ruined Boys Home"), go to the front door and face right. Let Carnage hit you and you'll end up in a secret room. Remember to use a daggerhead or the sonic gun to kill him.
When the second level begins, don't climb up yet. Instead, stay on the floor and go to the far right. Here you'll see a door with a window to the right of it. Go to the north-west of that window and fall into it to find a secret room!
In the level "RUINED BOYS' HOME", choose Spiderman, get in front of the main doors, face right, and let Carnage hit you. It will take a few tries, but it's worth it.
At the 2nd hero select screen, choose Venom. Then, at the level "FANTASTIC FOUR LAB", stand in front of the first left-pointing arrow and try a few jump kicks to enter a hidden room.
At the third hero select screen, choose Spiderman. In the level "POLICE STATION," jump into the jail cell with "27" above it.
At the level "MANHATTAN ROOFTOP", walk up to the skylight, and hold in right. When you can't move anymore, hold in left and then quickly hold in right instead, and jump. You will do a back-flip somersault onto the skylight and break through it to a secret room.