Use the Add Account button (it's the plus) to add an account to the list.
Select an Account Type. CalDAV and Google are best for Calendar publishing and subscription while the Omni Sync Server, MobileMe, and WebDAV are best for syncing.
Each ask for your account credentials. Once you've entered those, use the Test button to make sure everything's running smoothly. (If not, OmniPlan helps you identify the problem with a bit of red text next to that button.)