You can download Zotero on the Zotero download page. Be sure to also install a Zotero Connector for your browser.
Open the .dmg you downloaded and drag Zotero to the Applications folder. You can then run Zotero from Spotlight, Launchpad, or the Applications folder and add it to your Dock like any other program.
Run the setup program you downloaded.
Download the tarball, extract the contents and run
zotero from that directory to start Zotero.
For Ubuntu, the tarball includes a .desktop file that can be used to add Zotero to the launcher. Move the extracted directory to a location of your choice (e.g.,
/opt/zotero), run the
set_launcher_icon script to update the .desktop file for that location, and symlink
ln -s /opt/zotero/zotero.desktop ~/.local/share/applications/zotero.desktop). Zotero should then appear in your launcher.
Alternatively, Linux users may wish to use a package for their distro rather than using the tarball directly. Note that such packages are built by third parties, and we can only provide support for the official tarball.
Zotero should update itself automatically by default, or you can go to the Help menu and select “Check for Updates…” to check for updates manually. You can also always manually install a new version of Zotero over your existing version without losing any data.
If you're currently running Zotero 4.0 for Firefox and wish to upgrade to Zotero 5.0, you'll need to install Zotero 5.0 from the download page along with the new Zotero Connector for Firefox. See A Unified Zotero Experience for more information.