I would like to make a collection which returns my custom dexterity type if it's "featured" field is true. I have added this field to the catalog... is there anything else I need to do to be able to see it in the collection search terms?
When you say "I have added this field to the index" you mean to the portal_catalog?
If yes, you must also configure that index as a new collection criteria. If you are using new style collections you need a Generis Setup import step registry.xml. See plone.app.querystring package: https://github.com/plone/plone.app.querystring/blob/master/plone/app/querystring/profiles/default/registry.xml