A book chapter from an edited volume is entered as a 'Book Section.'
To add a book section to Zotero Standalone:
Click the green new item button and select 'Book Section.'
If 'Book Section' does not appear at first, select 'More' and then 'Book Section.')
Enter the chapter title in the 'Title' field
Enter the book title in the 'Book Title' field in the Info pane.
Click the plus sign (+) sign on the author line in the right column to add editors or additional authors.
If you click the triangle to the left of the new author field you will be able to change it to an editor.
If you need to change an existing citation's item type to a book chapter, select the citation, click on existing item type in the info pane (to reveal a drop down menu), and choose 'Book Section.'