I don't think I ever knew how to find the outbox either, but I just figured it out now:
- Just above the message list, which would be the inbox when you navigate there from the "My Messages" button, there is a line of three pop-up menu buttons titled "Messages", "Actions" and "Preferences". "Messages" will be highlighted in orange.
- Move the mouse over the "Messages" item and a pop-up menu will be revealed. "New Message", "Inbox" and "Sent Items" are the available functions in the menu. Clicking "Sent Items" will change the message list to the outbox.
Neat-o, huh?