I've only done this on the Mk2, so don't know if it still applies to the Mk1/Mk3.. but if you carefully prise out the Nikko symbol in the centre of the sidewheel you'll be able to remove one of the rubber o'rings. That will free it up a little bit. Then just keep turning the wheel to free it up a bit - mine used to get a bit stiff if I hadn't used it for a while (Oooo errrr missus)
There's another o ring in there as well, but you might not need to remove that after taking the first one out.
Ratinator beat me to it..