Having problems joining Mac to domain



My domain has a "companyname.local" DNS name on Windows 2003.

I am trying to connect a Mac OSX Server to talk to the Domain it isnt recognising my DNS name, even though it recognises other DNS names.

I have read that this could be because i have used ".local" rather than ".com"

Is this really a problem, and could someone point to a tutorial on the best way to join the Mac to the Domain.