Hi hshen,
The Beta 2 help lists the commands incorrectly and this is fixed for RTM. Here are the commands you may want to use (depends on the situation):
To use the Exchange Management Shell to grant full access permissions for a particular mailbox, run the following command to add the permission directly to the mailbox:
Add-MailboxPermission "Mailbox" –User "Trusted User" –AccessRights FullAccess
To use the Exchange Management Shell to grant receive as permissions for a mailbox database (and thus allow access into all mailboxes within the database), run the following command to add the permission to the mailbox store:
Add-ADPermission –Identity “Mailbox Store” –User “Trusted User” –ExtendedRights Receive-As
I hope this helps.
Ross