I sent a book to a member and he emailed me back that he had received it in good condition but Paperback Swap won't allow him to mark it received and refuses to give me my credit. They made an error and absolutely refuse to fix it. What can you do? I did everything right and am being punished for their mistake.

Whose mistake? The only way that I know of where the receiver can't mark a book received, is when the sender forgets to mark it mailed. Did you mark it mailed?

If you did, then it sounds like a glitch. You can contact PBS using the link on the bottom of the page. There is no reason why they wouldn't help you in the case of a glitch.

If the book wasn't marked mailed, or went "lost" in the system, it will no longer appear on his page for "books en route to me", so he may think he can't mark it received.

He should go into his Transaction Archive, under My Account, find the swap in question, and use the "Was it received?" button on the far right to mark it received, which will also grant you, the sender, your credit.

If you didn't mark it mailed and he received a 2nd copy-he doesn't have to give you a credit. 

(not saying that's what happened-just offering it as a possibility).

That is what popped into my mind...can't mark it  arrived, if it wasn't marked as sent...

However, to answer the last part of your question, you can go to "Contact Us" down on the bottom row of links, under the Company header.