Written confirmation of what's discussed above:
Prosper has recently made some changes to the payment options. In order to make an extra payment to your loan, you must first schedule a regular payment. In order to make a payment click on My Account > Borrowing > Make payment, then you’ll see a box to type in an amount for the “Additional principal” payment.
For future reference, your Prosper support case number is 00xxxxx.
Sincerely,
Prosper Customer Support