On the underside of the bottom of the bed are mounted blocks that hold the bed on the bolsters and coupling pole. I usually use scrap lumber for this because it's out of sight, generally 1.5" cut-offs of 2x4s.
Attaching them is easier with 2 people. To set them, place the bed on the assembled running gear, and holding the blocks butted in place one at a time screw them in.
When this is done you can lift the bed off as needed. The bed doesn't pop-off in use (gravity is our friend) but if you want to be able to lift up on the bed and have it stay attached to the running gear make the front and rear blocks taller and drill through for clevis pins.