Click the "gear" icon in the top right corner and select "Add mail account" from the dropdown menu. Enter your email address for the account you'd like to add and your password. Then click "Add".
Your email account should appear under your current email folders.
Tip: if you have multiple email accounts integrated you can choose which email address you send emails from. When composing, replying or forwarding an email just click your email address in the From box and select which email address you'd like to send the email from.