How often have you seen a picture of a US President and his wife, standing at the entrance of Air Force One and waving? That's actually the special entrance to the plane. Only the President, his family, and VIPs get to board the plane that way.
For everybody else, there's a separate entrance at the back. The idea is that all the top-secret facilities are in the front half of the plane, and so only the VIPs get to see them.