What exactly is the customer claim form for Energy Future Holdings bankruptcy and how do I fill it out?

If Energy Future Holdings owes you money for something, then you use the Proof of Claim form to tell the bankruptcy court how much the company owes you. Then someday if there is money left over to pay claims with, you may get a portion of what Energy Future Holdings owes you. If they don't owe you any money at this point, then you are free to ignore the claim form that you received.