The Touchdown pens are unique in that the ink sac is compressed by air pressure.  To fill, unscrew the knob on the end of the barrel and pull out the filler tube - it comes out about 5 cm.  Immerse the nib in ink and push the filler tube back down again fairly quickly.  Air pressure compresses the sac and then leaks away to allow it to re-expand.  Wait a few seconds for the ink to be drawn in.  Screw the knob back, then lift the pen out of the ink and clean the nib off with a tissue.  Done.