download iPad User Guide for iOS 5 from the iBooks store link.
Both are free and contain everything you ever wanted to know about your iPad and iOS 5
If you are asking how to add another mail account to the iPad, that is done in the Settings App.
If you mean adding another mailbox in an existing account, in addition to the Inbox and Deleted box, I don't think that is possible.
P
I'm new to iOS 5.1. How do I add a new mailbx to my list of mailboxes in Mail?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic