I cannot download the Comcast/Xfinity Norton Internet Security for Mac. From the Constant Guard Internet Security by Xfinity page, clicking on "Get Norton Internet Security" or clicking on the Apple Mac icon takes me to "http://my.xfinity.com/errors/unauthorized/?lang=en&ui_style=light" webpage which says, in part, "Sorry, we couldn't find that page..."
I tried this on TWO separate Macs: One running High Sierra (which might not be supported yet, according to the System Requirements) AND one running El Capitan (which already has NIS for Mac installed).
When I contacted Comcast Chat Internet Support, rep gave me a number to call for their "Norton Specialists...who can help with settings and installation..." I will try this in a little while (they open at 9 AM EST) and report back.
I scroll to the bottom of the page and click the link that says - "Get Norton Internet Security for Mac"
The page proclaims "Get Norton Internet Security for Mac" and features a red rectangle with white lettering that says "GET NORTON INTERNET SECURITY"
Clicking that link takes me to a download page with an activation PIN.
Please check https://constantguard.xfinity.com/products-and-services/norton-internet-security/ for the download link and instrcutions for mac. The current version of Norton Internet security for mac is compatible with High sierra. The system requirements have not been updated and I have already informed the same to the concerned team. Please try the download link and let me know how it goes.