When you install Zotero for Safari you will see a "Z" to the left of the address bar. This is different from Chrome and Firefox where the icon (e.g. folder or paper) appears to the right of the URL in the address bar.
If you don't see the "Z:"
- Restart your browser.
- Check to make sure the extension is installed and enabled.
- From the menu bar: Safari -> Preferences -> Extensions
- Make sure Zotero Standalone is open.