When you go to install Microsoft Exchange 2010, you need to prepare your AD first. MS has a good article on how to prepare the schema here.
The essence is that you need to run the following commands. If you want, you can skip 1. and 2. as they will run when you run 3. The advantage of running separately is tracking the changes and replication to your AD.
1. setup /PrepareLegacyExchangePermissions or setup /pl
2. setup /PrepareSchema or setup /ps
3. setup /PrepareAD or setup /p
4. setup /PrepareDomain or setup /pd
You only need to run this on any child domains under the domain you ran 3. on.
Another thing to note is that you MUST have a 64bit box on the domain that you run these commands from.
I ran into a client that didn't have an x64 machine on his parent domain, and was wanting to deploy Exchange 2010 to the separate client domains, but couldn't without loading up a x64 server on his parent domain.