I do agree with Loony Magic, that Bill told Ron, and Ron in turn was able to tell Harry about Shell Cottage. Also, Kreet has it right that Peter Pettigrew is able to tell Voldemort where the Potters are hidden. However, I would like to point out that Pettigrew was the Secret Keeper, but Ron was NOT. Ron was simply told about it. Also, if you notice, Ron never actually tells Harry the LOCATION of the cottage. Just the name of it.