Configure Project Server 2010 to work with Exchange Server 2010

Today I stumbled on a problem when trying to configure exchange server 2010 to work with project server 2010 task.
Cannot bind argument to parameter 'Identity' because it is null get-exchangeserver.DistinguishedName

Searching through google i found this http://spblog.oasisskinandbody.co.za/2011/12/add-adpermission-cannot-bind-argument.html#!/2011/12/add-adpermission-cannot-bind-argument.html

This command will configure all exchange server to work with project server 2010

$CAS = get-exchangeserver | where { $_.Name -match "<Specific Server Name/IP>" } 
$CAS = get-exchangeserver | where { $_.ServerRole -match "ClientAccess" }
$User = "<domain>\<user>"
$CAS | foreach-object {Add-ADPermission -Identity $_.DistinguishedName -User (Get-User -Identity $User | select-object).identity -extendedRights ms-Exch-EPI-Impersonation}