You have to design an ad first, so that you can select it when you set up your campaign. If you don't have an ad (of the correct dimensions for the site that you're advertising on), the screen that you see is quite misleading. It should say "you can't advertise on this site until you have created an ad of xxx by yyy pixels".

PS: Please don't advertise on Quezi.com, whose readers are not your target market.