using AD authentication

Hi, I am trying to use AD authentication from my mac computer. I was able to bind successfully on Active Directory plug-in. But I don't my mac login is authenticated against AD at all. (I have the same user name both in AC and MAC but they have different passwords. I can login into my mac using the mac password but not the AD user password) I researched a whole day yesterday about how I can set up AD authentication but it sound like AD plug in should be a straight forward setup. Is there any additional step needed to turn on AD authentication? any help would be appreciated. thanks, Yoshi