We're in the process of updating the documentation for Zotero 5.0. Some documentation may be outdated in the meantime. Thanks for your understanding.
What version of Zotero do I have?
You can check your Zotero version by selecting About Zotero from the Zotero menu (Mac) or Help menu (Windows/Linux) or, for Zotero for Firefox, from the Extensions tab of the Firefox Add-ons pane. Make sure you have the latest version listed on the changelog.
You can upgrade to the latest version of Zotero by clicking Download on the front page of zotero.org.