To access a delegated user follow the steps bellow :
1. Inside Gmail, Click the "Orb" in the top right corner that has your profile picture in it.
2. Upon clicking it you should see any mailbox delegations you have been given permission to, upon clicking one of these you will open another tab with with their Gmail.
Please Note, if you do not see the delegated account right away, please clear your cache/cookies in google chrome, here's how to Clear Cookies in Chrome