If you click on "settings" to the right of the thick blue heading line - "edit signature" is one of the options in the "my settings" area to the left of the screen.
People do not get this option straight away. I believe that it is dependant on both the length of time since joining the forum (which should be more than exceeded some years ago) and the number of postings... I am not sure of this level but suspect that it is about 10 postings...
If you do not get the "edit signature" option - I suggest that you make a few postings and look again.