The "Decks" page is where all the decks within the organization live. An organization may have many decks but users will only see the decks they have permissions to access. When an organization only has one deck users will be brought straight to their "Presentations" page after logging in which is how the "Decks" page tends to get overlooked.
(Note: You can tell you are on the "Decks" page if you see the "Decks" title above the search bar.)
How do I get to the "Decks" page from the "Presentations" page?
From the inside of a deck, on your presentations tab, click your organization's name in the top left corner next to your deck logo.
The "Decks" page should be easier to find if you have access to multiple organizations and decks. User with access to multiple organizations are brought to the "Organizations" page after logging in.
How do I get to the "Decks" page from the "Organizations" page?
From the "Organizations" page it will be fairly simple, select from the list a organization whose "Decks" page you would like to see. Upon selecting you will be brought to that organizations "Decks" page.