Display Suite layouts not available for field collections Edit\Add forms
If you are using 7.x.1, you need rel module to edit forms from other entites than node type, like field collection, user, comment, etc.
If you are using 7.x.2 you don't need that because this branch offers this feature out of the box.
For editing forms, you must go to Manage fields of your entity.

edit: just to note that rel will stop this from working on the 7.x.2 branch.