If it's like the AP100 I have that's held together (literally) with duct tape, coat-hanger wire and rubber bands...
Stand the vac on its end.
Open the bag door and take out the bag.
Peel back just a bit of the rubber bumper strips on the sides.
Grab the ends of the now-exposed plastic slider strips with some pliers and pull straight up.
MasterBlaster
April 2006