you have to be a team founder, or a captain on that team.
That person has to be on the team roster as well.
Click on the "admin [ladder name here] ladder" button, and scroll down to the bottom of the page.
You should see a dropdown box that has a all the players in it, and a button under that box that says "promote to captain" (or something to that affect)
highlight the player in the box, and click the "promote to captain" button.
there you go, a new captain is born