Access the data field design mode by selecting Edit > Design Mode from the application menu. You can also access design mode by clicking Design Mode on the toolbar.

The active database tab will govern whether you open the data field design for the main, calendar or results database. Each of these databases has its own set of data fields which are independent of the other databases.

If you have two database tabs open for a particular database type (e.g. main), you can make either tab active to access the main database data field design.

If you have two of the same database type within a project (e.g. calendar), then only one of them will be active at any one time. It is the data fields for the active database that you will access when you open data field design.