If you have been added to a brewery account as a manager, you will access the brewery via your personal account. To enter the brewery page, log into your personal account on untappd.com in a web browser (brewery accounts aren't available on mobile), and then use the search bar in the upper right hand corner to go to your brewery page.
At the top of the page you will see a yellow banner that says, "You are a manager of this brewery. Use as this brewery?" Click the "Use as this brewery?" part that is hyperlinked and it will log you into the admin side of your brewery page:
Once inside, you can perform all functions as normal.