There is no such functionality built into Directory Plus. However, Sabai recommends the Post Category Filter which provides this functionality on the backend (WP-Admin).
Unfortunately, this does not help with the front end user submissions.
Category: Search and Filter
How can I set a search radius to the widget area?
This is being added in 1.2
Can one have custom fields included in search queries?
- Yes: Directories –> [Your Directory] –> Content Types –> Listing –> Edit –> Search –> Keyword Search –> Extra Fields to Include in Search.
- Not all field types support this feature.
How can one add filters in a sidebar to refine the list of listing results shown?
Use the Directories: Filter Form widget.
Is it possible to limit certain filters only to specific categories?
No, but this is planned for a future version, probably 1.2.x.
Can one search across multiple directories using the Directories Pro search?
No. However, standard WordPress search will search across all directories and by using a plugin such as Relevanssi one can index additional fields to be included in the WordPress search.
Sabai does intend to add the ability to search multiple directories through a tabbed search interface, but this will still not allow one to search all directories from a single search field.
Is there a way to set the default search radius?
Directories –> [Your Directory] –> Content Types –> Listing –> Edit –> Search –> Location Search.
How are listings sorted?
They can be sorted in a number of ways.
Is there a radius search filter?
Yes.
Can I add a custom field to the search bar?
One can add fields to a filter but not to the search, see documentation for more on filters.