The editor for a DB Definition is broken into two main windows - the Main Screen and Designer.
The Main Screen is the initial window that opens when creating or editing a DB Definition. This screen allows you to input the main parameters of the DB Definition. These should be populated prior to opening the Designer window.
No matter where you have changed the configuration within a DB Definition editor windows, that configuration will not be saved unless you click the Save button on the Main Screen.
The Designer window allows you to define the data structure to represent the data in your database. This provides the Flow application the "meta data" for it to understand your database, and enables reading and writing of data to the database. The Designer window is accessed by clicking the Design button on the "Main Screen".
The Designer opens as a separate modal window, this means you are unable to use other windows of Flow until you close this window. If you minimise this Window you will find that you cannot use the other windows of Flow.