Open up Exchange System Manager by going
Start -> Programs -> Microsoft Exchange -> System Manager

Drill down to your Mailbox store (Servers -> SERVERNAME -> First Storage Group -> Mailbox Store) and right click then choose properties. Click on the Security Tab.

Click on the'Add' button. Then choose the 'SSWTeamCalendar' user.

Make sure that "Full control" is ticked for this user.

This step is now complete. Click 'Back' and select 'Step 3'.