You can locate the settings you'd need to use to add a mailbox to a mail client (such as Outlook, Thunderbird, Mail for Mac etc) by logging in to your cPanel, and then clicking on:
EMail accounts > Connect devices (on the same line as the mailbox you'd like to add to your mail client)
In the page that then loads you'll see a "Mail Client Manual Settings" section that contains the settings you'd need to enter in your mail client.
The password for the mailbox isn't displayed on this page, if you don't know the password for the mailbox in question you'll need to reset it. You can find guidance covering this here:
https://my.netnerd.com/knowledgebase/10120/How-to-change-a-mailbox-password.html
