[FatalError]

very simple:

* open iCal
* menu "calendar" > subscribe
* Calendar URL: webcal://sync.omnigroup.com/YourUserNameHere/OmniFocus-Reminders.ics

Since it is a log-in access you'd most likely be asked for your access credentials.

On the iPhone it's a similar process. I have not found a way of easily transferring subscribed calendars to the iPhone from the mac (shame on you, apple!).

You'd go to (on iOS5):

* Settings > Mail, Calendar, Contacs
* Under Accounts > Add Account
* Other
* Add Subscribed Calendar (on the bottom)

Hope this helps :)