This is an old post now, but for anyone else having this problem:
-login to the patreon page
-click "My Membership"
-expand the welcome post
-click "Mythical Monsters Patron Access on Itch.io"
-click "Connect with Patreon" on the itch page and "Allow"
It's a little confusing because he has two different links for each set, one links here and the other to download page.