Streak can be installed directly from the Chrome Web Store as an extension.
Open Streak's extension in the Chrome Web Store.
From the top-right corner, select Add to Chrome.
Confirm installation by selecting Add extension in the pop-up in the top-center of Chrome.
Refresh or navigate to Gmail.
Streak is available as an extension from the Mac App Store and can be installed from there.
Open Streak's extension in the Mac App Store.
Select Get and then Install Now.
Once installation is complete, click Open.
Click Open Safari Preferences in the Install Streak for Safari window.
Mark the checkboxes next to Streak and Streak Helper to enable them in Safari Preferences.
Refresh or navigate to Gmail in your Safari browser.