To enable the Developer tab on the Outlook Ribbon, navigate to the Backstage view (see "What is the Outlook 2010 Backstage view?") and select Options. In the left pane of the Options window, select Customize Ribbon. This feature lets you determine, to some extent, what components are visible in the Ribbon. The Developer option in the right pane is deselected by default, as Figure 1 shows.
Select the check box beside Developer to add the tab to the Ribbon. Figure 2 shows the Outlook Ribbon with the Developer tab selected.
Figure 2: The Developer tab on the Ribbon in Outlook 2010
You can further customize which options the Developer tab includes. Expanding the list under Developer by clicking the plus (+) symbol to show the default sections visible on the Outlook Ribbon when the Developer tab is selected: Code, Add-Ins, and Custom Forms. These sections can be removed from the Ribbon individually if you don't require all of them. Perhaps you're designing Outlook forms, but you don't need access to the Visual Basic for Applications interface accessed through the Code section. Removing an option from the Ribbon doesn't disable the functionality, but rather suppresses the visual path to launch the feature. For functionality that isn't needed, out-of-sight, out-of-mind is typically sufficient for busy users.
Not surprisingly, the settings for which tabs appear on the Outlook Ribbon are accessible through the Registry as well. The DWORD value to toggle the Developer tab on or off is HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Outlook\Options\DeveloperTools. A value of 1 indicates that the Developer tab is enabled and present on the Office Ribbon in Outlook 2010.