MacDock no longer accepts PayPal subscription payments. If you have created a subscription:
Login to your PayPal account and go to the "History" subtab of the "My Account" tab. Choose "Subscriptions" from the pull down "Show" menu and press the "Submit" button. Choose the subscription payment to email@example.com" and click on "Status". You will be taken to a Transaction Details page from which you may cancel your subscription.
You'll still have the option of paying your monthly hosting invoice with PayPal.