Log on to Forefront TMG server as an administrator. Start menu>All Program>Click Forefront TMG management>Expand Forefront Server>Right Click on Firewall Policy>Click New>Click Exchange Web Client Publishing Rule>Type Rule Name>Click Next>Select Exchange 2010 from Exchange version>check Outlook Anywhere and Publish additional folders on the exchange server for outlook 2007 clients. Click Next. Now follow the screen shots for rest of the configuration.
Once you click finish then apply changes.