To load a pack of film in an Impossible or Polaroid camera, slide the film door latch to open the film door. Push the film cassette all the way in, with darkslide up and plastic base at the bottom.
When you close the film door, the darkslide will eject from the camera automatically, underneath the Film Shield. The plastic Film Shield is designed to protect images from light as they develop – it should not be removed. Take the darkslide out from under the Film Shield and let the Film Shield roll back up. If the darkslide has not ejected, take out the film pack and re-insert it, making sure it is pushed all the way to the back of the camera.
If you're still unsure, watch Peter and Heinz do it below: