This is how I set mine up. I don't use active sync or Outlook
Click on the Settings icon
select "Mail, Contacts, Calendars"
select "Add Account..."
select "Microsoft Exchange"
enter the settings for the account:
email:
username@whatever.com
server: mail.whatever.com
username: username
password: [Zimbra Password]
description: your choice
use SSL: On (most likely)
Once the account is setup, it may take it quite a while to sync. You can also customize the account to enable/disable Mail, Contacts, and Calendar syncing.