Server 2007 :  note : change EX2007 to computer name, change @DONAIN to domain name

Enable-Mailbox iBackup@DOMAIN -Database "EX2007\First Storage Group\Mailbox Database"
Get-MailboxDatabase | Add-ADPermission -user <BACKUPUSER> -AccessRights GenericAll




Server 2010 : note: change @DONAIN to domain name

Enable-Mailbox iBackup@DOMAIN
Get-MailboxDatabase | Add-ADPermission -user <BACKUPUSER> -AccessRights GenericAll