Type "SJE" to activate/deactivate super jump. When you're in the air, press Crouch to jump.
You can adjust the jump's settings in SJ.ini (you can even do it while playing, just use Alt+TAB to switch between the game and CLEO folder), the archive has an image that will help you understand how it works. You need to deactivate the script to apply new settings.
Installation:
1) You need to have CLEO 4 installed;
2) Drag SJ.cs, SJ.ini into GTA SA\CLEO.