Click on the icon circled below. A box will open in which you type in the ISBN, DOI or PMID for the item you want to insert. The item will be added to your Zotero library.
To insert an item manually, open either the standalone or Firefox version of Zotero. Click on the New Icon button (circled below), which opens up a dropdown menu listing source types (book, book section, journal article, newspaper article, etc. A window pane opens on the right with slots for you to enter all relevant bibliographic information: the example shown here is for a book.