Admin console> domains> select a domain> configure authentication button
info here:
http://www.zimbra.com/docs/ne/latest/administration_guide/5_Zimbra_LDAP.5.1.html#1036410
I would suggest making your personal account/an account that's already in the AD as an administrator before you are fully on external auth-
If you can't login to the admin console after that:
1) is there a corresponding user for 'admin@domain.com' in the AD?
2) if desired, enable fallback authentication
zmprov md zimbraAuthFallbackToLocal TRUE (or zmprov md yourdomain.com zimbraAuthFallbackToLocal TRUE)