I've looked around a bit in Outlook, and it looks like the command is only available when you open an email message in a new window. It's not accessible through with the reading pane in Outlook's main window.

Try this to see if you can add the command to a custom group in the ribbon:

1. Double-click on any message to open it in a new window.
2. Right-click on the blank area of the ribbon to the right of the other icons.
3. Click Customize the Ribbon.
4. Under the "Choose command from" drop-down, select All Commands.
5. Find "Edit Message" in the left-hand box, and click it to highlight it.
6. Click the "New Group" button below the left hand box. This should create a "New Group (Custom)" group under the Message group. You can rename it if you want.
7. With "Edit Message" and the New Group highlighted, click the Add button.
8. Click OK.

The Edit Message command should now appear on the ribbon.

If the Edit Message command isn't available under All Commands, then I think there'd have to be something wrong with your installation. I'm running Office 2010 Pro Plus as well, and I didn't need to do anything special to access the command.

Hope this helps.

-SW