Splitloop, if you open the Sync pane of OmniFocus' preferences, is the "Sync with: Omni Sync" option selected? If so, all you need to do is check the "Publish Due reminders as a calendar" box that Lizard mentioned; OmniFocus will take care of the rest.
If you have one of the other sync options (including "None) enabled, feel free to call (800.315.6664) or
email our support ninjas and we can help you set this up.