Don't forget to sync!
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.
Manual Entry
You can also add references manually. You can do so by clicking the "New Item" button, which is a green circle with a white plus sign inside: 
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.
Zotero Connector Browser Extension
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.
- Depending on the extension you are using, look for an icon to the left or right of your browser's address bar. It often looks like a capital letter "Z", but it can change based on the type of resource you have open. Sometimes it looks like a book or a piece of paper, for example.
- Click on the icon to save the citation(s).
- In the lower right-hand corner of your web browser, a dialog will pop up displaying the citation information for the particular resource you saved to your library.
- If you are currently working in a specific collection (that is, a collection
highlighted in the left column rather than “My Library”), the reference(s) will be
copied to that location as well as your main library.
- If you are currently working in a specific collection (that is, a collection
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.
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.
Saving Screenshots
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.
- On the page of the resource you want to save, click the screenshot icon in your browser extension of choice. The icon will appear as a blue sheet of paper with a green/white plus sign.
- You'll need to go to your library to add citation information for this 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.
Identifier
Zotero can add items via their ISBN number, Digital Object Identifier (DOI), and PubMed ID.
- With Zotero Online or Standalone open, click the "Add Item by Identifier" button. It looks like a wand with a green/white plus sign icon.
- Type the ID number and click "OK". You can also paste or enter a list of such identifiers at once.
- Tip: Press Shift+Enter for a larger box.