McKibben is President and Co-Founder of 350.org. He's in charge. Presidents of nonprofits usually know who funds them. He doesn't seem to want to say. To her credit, Karyn Strickler, the interviewer, presses him on it and expresses her incredulity, even though she is sympathetic to 350.org's mission.
What is with these environmental organizations (Center for American Progress is another one) who expect transparency from their foes but won't provide it themselves?
I'm not sure exactly when the interview was shot -- the video's stats start the week of 12/7/10.