you are on that screen.. arrow keys are move, and ctrl is jump, alt is to use the pogo stick, spacebar pauses the game, and shows your status, and alt+ctrl is shoot. the dudes that are green with one eye are the only ones you can jump on, everyone else you have to fall mercy to, avoid, or shoot.
edit: walk up to the big blue thing blocking your path and press the ctrl button to enter the level.