Accounts are activated automatically after payment has been received from the payment processor. This requires that Paypal sends a message to our servers indicating that the payment was successful. In very rare circumstances this message between servers can fail.

However its more likely that you have sent an e-check payment (a payment made from your bank account) which won't clear for a few days (unlike a credit/debit card which is instant). If this is the case please be patient until Paypal processes the payment.