Hi Mike
In exchange 2007/2010 you can not create mailbox using ADUC while creating new users. You can create new AD users and mailbox using exchange console and it will replicate to AD also. For the users which are created using ADUC and not showing their exchange tabs.Go to
Receipeint configuration->Mailbox->New Mailbox->Select existing user-> find that user and creat the mailbox.
Tahir