receive a book that is the wrong one, and I would like my credit back, After I mark it as  rwap do I have to ask the sender for the credit or does PBS send her a pm??

I believe you have to ask for the credit.

Bertie, how is the book  wrong?  Wrong book--wrong binding/version?  or Wrong book--wrong title?

It was a wish list book that should have been Large Print and it was regular print

• Wrong Book - Wrong Binding or Version
  • Again, this means that title/author DO match but the book does not match the listing's ISBN and/or binding type and/or it was listed as Large Print and the book is not Large Print.
  • You will have a text box - the text you type in will be included in an email, and sent as a Personal Message, to the sender.
  • The book IS marked received for this problem type because if a sender Posted the book she or he had incorrectly, it is likely that she or he does not have a second copy and can't send you the version that matches the listing.  
  • You will have the option to request a credit refund - this is checked by default for this problem type.  If this option is checked, the email the sender gets will include the request for a credit refund. If you uncheck this option, the email sent to the sender will NOT mention or request a credit refund.
  • If this was a Wish List book (and there are no other copies available), you will go back in your previous place in line for the book.  If there are other copies available, the book will go onto your Reminder List.

What Hunter said

TPTB give a minimum time you must wait for a reply.  IIRC, there's a new option (button) that if the RWAP sender does not answer within a certain time frame you click the button and a more official PBS PM is generated...reminding the sender they must reply.