If your references aren't showing up, you may need to sync your library.
For information on saving PDFs and attachments, see the "Organizing Your Library" page.
Browser Extensions
If you installed Zotero for Firefox or any of the of the browser extensions for Safari and Chrome, you are able to save citations automatically from within your browser to your Zotero library.
On some web pages that list information about multiple items (e.g. a list of database search results), Zotero will show a folder icon. Clicking this folder icon will open a window in which you can select the items that you want to save to your library.
Manual Entry
To add references manually, follow the steps below.
Identifier
Zotero can add items automatically using ISBN number, Digital Object Identifier (DOI), and PubMed ID. Follow the instructions below to save by any of these identifiers.
Saving Webpages
Sometimes Zotero can't recognize the type of resource you are viewing in your web browser. Even if Zotero can't automatically capture citation info from a certain web pages, you can still add them to your library.
Taking a screen shot saves a copy of the webpage to your computer, which means if the page is removed later or if you're offline, you'll still be able to view your copy.
Tip: You can also change your settings to automatically take screenshots of webpages. See the "Organization, Attatching Files, and Screenshots" tab for more instructions.
Zotero provides an official list of compatible sites, but keep in mind that many other sites not included on this list work with Zotero as well.