Although I've praised Zotero for it's intuitiveness, one of its less intuitive features, and a regular point of confusion for newer users, is how to add editors (the human type) to an entry - necessary information for a book section or edited book.  The drop down menu does contain a Book Section option, but this gives no obvious choice for adding editors.  The trick here is to add the editor(s) as you would with a regular author, then select the small arrow to the left of Author.  This will open a menu that allows you to set the individual's role (e.g., Editor, Series Editor, Contributor).  Once this is done, Zotero will take this cue to work out what type of reference it's dealing with when formatting your citation for your reference section.
2. Entry with Identifier.  An easier way to add an item is to use the integrated Zotero database.  If you have an object identifier (e.g., DOI, ISSN, PMID), you can enter it as pictured in Fig. 11.