Check this link out about configuring CIFS, I know it is Data ONTAP 7.1 but I am pretty sure the prerequisites still apply. The document goes into a lot of detail around configuring AD and connecting the filer. I think you are probably hitting the old "your time is more than 5 minutes out of sync" issue. This means client authentication attempts to the filer may fail. Recommend using NTP time too if you can, this way your filers and AD servers get their time from the same source.