I have a Nikon Super Coolscan 4000ED (or at least I did until the FireWire went funny and I gave it to the Nikon repair centre where it's been for the last 6 months).
to scan the many rolls of Advantix we have, I purchased the Advantix scanner adapter. one gotcha with Advantix is that you cannot unwind and re-wind it very many times. in fact, two or three times is the maximum. each time you unwind/rewind it, the film tightens until it simply doesn't unwind anymore. the only apparent solution to this was to get someone to duplicate the film into another advantix cartridge.
you *can* get the film out of the cartridges though (back to your question). you need a small screwdriver to press down in the side of the 'door' to turn it and open it. with another larger screwdriver, turn the main axel of the cartridge until the film pokes out through the door.
note that when you wind the film back into the cartridge, you need to turn it in until the indicator moves to position 4 indicating that the film has been developed. if you don't do this, developers and scanners may not be able to open the cartridge with their usual methods.
the negative frame size is very small; approx 18mm x 32mm (I just measured one)