The Designer window allows you to define the data structure to represent the data in your file. This provides the Flow application the "meta data" for it to understand your file, and enables reading and writing of data to the file.
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 File Definition. This screen allows you to input the main parameters of the File Definition. These should be populated prior to opening the Designer.