remove the gear nobs and take the cover off so you can see the top of the box.
undo the 4 pins holding the selector housing on and pull it out.
then you should see a ball and socket where a grub screw locates and locks onto the selector shaft, if you lost 2nd and 4th which is a pulling action not pushing ( 1st, 3rd,5th ) then its quite possable that this has come undone.
dan