If you have custom fields assigned to all categories you may want to display them always in the Filters/Search Module. In this case users' won't need to choose any category to reveal the custom fields that are applied to all categories.
First, in back end, go to the DJ-Classifieds Search module and make sure that "Show custom fields" option is enabled.
Then go to the extra field that you want to be assigned to all categories (you can assign a field to all categories by opening it in back end [Edit tab], scrolling down and clicking the "Add to all categories" link.
Once the field is assigned to all categories, open the "Search" tab in the very same extra field and select "Show in search on start" to "yes" and save the extra field.
Now this extra field will be visible when no category is selected in the module.