Sorry I don't know what I am doing,When I follow your good direction multi bit says that my wallet does not contain that bitcoin address?
Find your address in MultiBit. This can be done by going to the Request tab. Your address is at the bottom of the 'Request' page, under the text 'Your receiving addresses'. Copy your Bitcoin address by double clicking the text below 'Address', then copying by doing command+C on mac and control+C on Windows. This is the address that's in your wallet, and the one that I think you want to use on the website. Use this address when you're asked to give your Bitcoin address to the sign the message in MultiBit.