How to set the mainpage of a custom post type?

When you register the custom post type you need to include has_archive set to the slug where you want the archive. You also need rewrite set to the slug prefix. It’s probably easier to understand the code rather than my wording of it:

Your code should look something like this now:

register_post_type('conference',
    ...
    'public' => true,
);

You need to update it to:

register_post_type('conference',
    ...
    'public' => true,
    'has_archive' => 'conference',
    'rewrite' => array('slug' => 'conference'),
);

You may need to use unregister_post_type first, to clear it out, and then re-register with your updated function. After the new version is registered, visit your permalinks page to update permalinks, and you should then have both an archive at myurl.co/conference and individual posts at urls like myurl.co/conference/program-item-slug.