Advanced Custom Fields

I’m trying to do the same thing, to add the possibility to search in a specific custom field…

It can be done. You need to edit the template by clicking on the “Edit template & settings” of your desired section. e.g.

In the post type search, I edited the “post” template to display “manufacturer” and “composition” by following:
<a href="{post_link}">{post_image_html}<span class="sf_text">{post_title} </span><span class="sf_small">Manufactured by {custom_field_manufacturer}, composition {custom_field_composition}</span></a>

For custom fields: {custom_field_(YOUR_CUSTOM_FIELD_NAME)}

But this is a solution to display custom fields on the search results, not to search custom fields ??