The Designer window allows you to define the data structure to represent your web service. This provides the Flow application the "meta data" for it to understand your web service, and enables reading and writing of data to the web service.
The Designer window is accessed by clicking the Design button on the "Main Screen". The Main Screen is the initial window that opens when creating or editing a Web Definition. This screen allows you to input the main parameters of the Web Definition. These should be populated prior to opening the Designer.