To manually add an item to your Zotero library, click the green “New Item” () button at the top of the center pane, and select the desired item type from the drop-down menu.
An empty item of the selected item type will now appear in the center column. You can then manually enter the item's bibliographic information via the right-hand pane.
Zotero can add items automatically using their an ISBN number, Digital Object Identifier (DOI), or PubMed ID. This is done by clicking the Add Item by Identifier button () in the Zotero toolbar, typing in the ID number, and clicking OK. You can even paste or enter (press Shift+Enter for a larger box) a list of such identifiers at once.