Hi,
1. Verify that you have Autodiscover working correctly with your Exchange Online: help.outlook.com/.../cc950655.aspx
2. Make sure you have the latest service pack for Office 2010: pcsupport.about.com/.../office-service-pack.htm
3. Add an additional account in Outlook 2010 using the Autodiscover method (Enter E-mail address to find server settings)
Best regards,
Jesper Ståhle