Configure the iPad to use Exchange ActiveSync to connect to the Zimbra server. Then, it will sync the Calendar, Tasks, Contacts, and E-mail into the standard apps on the iPad. Then just use the standard mail, calendar, addressbook apps on the iPad. Works extremely well (we use that here).
If Comcrap doesn't give you ActiveSync access to your comcast.net account, then complain to them, not Zimbra.
Alternatively, on the login screen, select the "mobile" or "html" option, instead of the "advanced (AJAX)" option. That will give you a single-click interface.