Display Suite: Panel layouts in doesn't appear in "Manage fields"
The Manage Fields tab is used for arranging fields for the Node Add/Edit form, as well as to Add/Edit/Remove fields to the Node/Entity.

If you want to use Panels for the layout of your Node Add/Edit forms, then you would do this through the Panels Interface by enabling and editing the Node add/edit form Panel that comes included when you install Panels. You would create variants in this Panel to handle the layouts for each content type you have. Hope that can help.