I had a similar problem; I downloaded it on a mac that didn't meet the minimum os requirement, then deleted it. Couldn't use the link again on the newer mac. Don't most people expect to be able to re-download this?
Go to the website (https://atom.io) and click Sign In in the footer. A download link will show once you are signed in.
Thanks! After Signing in it redirects me to my github page.
I had to visit atom.io again, click Sign in again and - voila - Download button appears. Very confusing.
Have you tried logging in, using the link at the bottom of the page?