In order to configure the Custom Fields, you have to create a field with its corresponding values and then associate it to a specific product.
There is nothing to enable in the module for this. Once the custom field has a value and is assigned to a product, it reaches Doofinder on its own. It is not indexed by default, though, so you have to add it:
- In the Doofinder Admin Panel, go to Configuration > Search Engines > click on "See indices" > Indices.
- Scroll down to Additional indexed fields and open the Select the fields to be indexed dropdown.
- Choose the field, press Save and reindex your catalog, so it is picked up.
See How To Index Your Feed for the detail. Once indexed, you can set it up as a filter under Search > Layer Settings > Filters.
This needs an up-to-date module. On older versions the fields are still selected in the module settings.