Check out this blog post to get mutt working with GMail on OSX (http://www.jstorimer.com/shells/2010/01/19/using-mutt-with-gmail-on-osx.html).
You can always use the '?' command to see a full list of mutt commands. This sheet lists commands that map to specific GMail functions.
Select messages (up and down): 'j' or 'k'
Page through message: <Space>
See a list of mailboxes: 'c'
List URLs: <Ctrl>-b
Mark as unread: <Shift>-n
Archive message: <Ctrl>-d
Sync local mailbox to GMail: $