First, you will need to generate an iCalendar link from your Zen Planner calendar. In your database, click on the Calendar icon along the left hand sidebar. Use the filters in the submenu to customize the calendar view to only show the staff members and items that you want to share.
After you finalize the view, go to the upper right hand corner and click on the Export to iCal button. This will launch the popup menu where you can access the iCalendar subscription URL for your current calendar view, and read a brief explanation of how to use the URL.
From your iPhone
Go to Settings and then to Mail > Contacts > Calendars. Next you will determine which calendar you are adding and then add to subscribed calendar. Finally, you will insert the URL address you took from the calendar and paste or type it in under server.