I just opened the bleed valve at let it drip through for a bit; until it looked pretty clean... then did it up and gave the pedal a couple of pumps; that seemed to work. - As long as there's fluid in the reservoir it will flow down-hill, and pumping the pedal seems to convince any air to rise back up to the top.
I did try pumping the pedal and fiddling with the nipple, but letting gravity do the work was easier in the end... and less frustrating.
Er... if it won't engage any gears, are you sure it's the slave cylinder that's not working? There's no chance that your fork has let go is there? Release bearing is in good condition? Plate hasn't fallen apart (that happened to me; I thought the clutch fork had gone, but it was actually a piece of the clutch plate wedged in the cover stopping it from releasing).