Manufacturer-specific custom fields from ThemeWare®.
Switch via the main navigation to the "Manufacturer" management of your shop ("Catalogues" => "Manufacturers")
Choose the manufacturer you want to edit
Scroll to the block "Custom fields"
Select the tab (=set) for your theme (e.g. "Manufacturers (ThemeWare® Modern Pro)")
Make the desired changes
Save your changes
You have now edited manufacturer custom fields.
Tip: If the navigation bar with the tabs (=sets) is full, you can scroll it using the arrows (left and right).