If references aren't showing up, you may need to sync your library (especially if you use both Zotero Standalone and and Zotero Online). To do this, click on the circular green arrow in the upper right corner of the program.
This blog post describes some options for extracting references from bibliographies you have already created and adding them to your Zotero library.
If you installed the Zotero Connector for Firefox, Safari or Chrome, you are able to save citations automatically from within your browser to your Zotero library.
Multiple Citations
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 where you can select the items that you want to save to your library.
Adding PDFs
Add existing PDFs to your library by dragging them into the Zotero pane. Most of the time, Zotero will search for the metadata for that PDF and create an entry for it. If it does not, check your Settings or Preferences. In Edit > Settings > General, check the boxes for "Automatically retrieve metadata for PDFs and ebooks" and "Automatically rename locally added files."
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.
Saving Snapshots
Taking a snapshot 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. It functions similarly to a screenshot.
Compatibility
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.
You can also add references manually, if the connector isn't working correctly. You can do so by clicking the "New Item" button, which is just above your reference list and is typically a paper icon with a plus sign in the corner.
From there, select the appropriate item type. If you hover over "More," that will give you many other options. After that, you can add the other metadata (like title and author) in the right column.
To import libraries from other reference management tools into Zotero, start by exporting the bibliographic data from your other software program. In Zotero, you will find the Import option under File in the menu bar. Browse to your file, select it, and click the “Open” button.
Zotero can import the following bibliographic file formats:
If you want to transfer entire Zotero libraries between different Zotero installations, you should use Zotero's sync functions, rather than the import/export function.