You probably need to turn the membership on once you've paid and assuming you sent to the correct address and not some scammer.
Pay to the Bitcoin address. Once your transaction has 1 confirmation, revisit this page and then use the form which will appear in order to wear your new membership. Your transaction will not be registered until it has 1 confirmation. Bitcoin network fees and exchange/wallet withdrawal fees will not count toward your payment.
It looks like the link posted in the thread OP's reffering to did indeed point to the correct page, so i can only assume he didn't pay a scammer.
Like hilariousetc said: did you revisit
https://bitcointalk.org/index.php?action=credit;promote in order to turn on your membership? Also, are you 100% certain you sent the right amount AND your transaction has at least 1 confirmation? You'd be supprised how many times new members think they can substract the fee from the output value (not saying that's the case here, just saying that these sort of things happen quite often).
Last but not least, double check if don't have one of these virusses that change addresses in your clipboard... Double check you payed the address that was shown on
https://bitcointalk.org/index.php?action=credit;promote