I am not a master in this type of stuff but let me see if I cna help you out.
Here is a link I found to match te type of "Supervisor" you need. This is the part that does the work. LM810M3-3.08
http://www.national.com/pf/LM/LM810.html
Next you will need this part: Which a google search reveals it is a red LED. L517URD-F
Seems to be fairly simple up front.
From the photo you provided you would wire it up like so.
- The # 3 tab on the supervisor is wired to the positive side of the battery terminal.
- The # 1 tab is wired to the Ground
- The #2 tab is then wired to the + side of the LED and the - side is then wired to the ground side of the battery.
Hope this helps.