The 1Password extension for Safari should begin downloading within a few seconds. Once the download finishes, double-click the 1Password.safariextz entry in your downloads list.
You can also find this file in your Downloads folder and open it from there.
Safari will ask you to confirm the installation:
Click the Install button. If the 1Password icon does not appear on the toolbar, click the Safari > Preferences menu, select the Extensions tab, and verify that Extensions are enabled and that the 1Password extension is enabled as well.