If you're saving a source that does not have an online presence or a unique identifying number, you can create an entry in Zotero manually. (Examples of when you might need to do this include citing proprietary software or email communications.)

Click on the far-left icon above the center column of your Zotero account; when you hover over it, it says "New Item." Select your type of source from the very long list that appears.

The Zotero program, showing the "New Item" icon and list of source types.

A blank entry will appear in your Zotero library, allowing you to enter any citation information you have. Not all the fields that Zotero provides may be required for your citation style. You may wish to refer to the style guide of the citation style of your choice.

The Zotero program, showing a blank entry ready for editing.

When you're finished adding the citation information, just click anywhere else on the screen and it will be automatically saved.

The Zotero program, showing an entry added manually.