2 – Select: “Customize Ribbon” in left navigation
3 – Select “Main Tabs” section (or make a new tab)
4 – Add a new Group. Rename the group to your liking (“Message Headers”, perhaps?)
5 – From the “Choose commands from:” menu on the left, select “All commands”.
6 – Scroll down to locate “Message Options” and add that to your new Group.
7 – OK the changes, and exit.
The long way to get to the same area in 2010 is to:
1 – Double click to open the message you want to see headers for.
2 – Select: File > Info >Properties