DBA Workbench and the Adabas Community Edition are for Windows, not the mainframe. This thread should be posted to Adabas Basics on Open Systems.
In Workbench:
. double-click a database to display its file list
. on the menu bar, click File → New
. enter the field specifications manually
If you’ve already created a DDM in Natural, or if you have the FDT from another database, you can replace the last (manual) step by clicking FDT on the menu bar.