It seems that you cannot access the Exchange 2010 Online Archive using Microsoft Outlook 2010 in the MSDN or TechNet versions of Microsoft Office Professional Plus 2010 RTM. This may be due to the product IDs provided on MSDN or TechNet.
The Online Archive is an Exchange 2010 feature that creates an optional associated archive mailbox for a user. Exchange move policies can be created to move content into the archive based on various critieria. Exchange 2010 RTM hobbled Online Archives by requiring that they exist in the same database as the associated mailbox, making their use limited. Exchange 2010 SP1 will improve Online Archives by allowing you to create remote Online Archives on a different server (perhaps with slower, cheaper storage). The Online Archive feature requires Exchange 2010 Enterprise Client Access Licenses (CALs).
The images below show the behavior in both Outlook 2010 Professional Plus (on the left) and Outlook Web App 2010 (on the right). Note the lack of of the Online Archive in Outlook.
I’ve confirmed this on several machines, running both Windows 7 x64 and Windows Server 2008 R2. I’ve completely uninstalled and reinstall Office 2010 Professional Plus with no effect. I also confirmed there is no change when using Outlook in Cached Exchange mode or online mode.
If you notice different behavior, I’d love to hear from you. Leave a comment below.