How can I leave negative Feedback

[daninausuk]

I have had a buyer that has brought a number of items from me and is refusing to pay for them. I want to leave negative feedback but there is no button for this. When I go to leave feedback it gives me the option to leave positive feedback, leave feedback later or report a problem. When I have clicked to report a problem they are saying that the web address for that area of the site is no longer avaliable. Please can someone advise how I can leave negative feebback!!!!!!

Thanks