Following is the sample code to read lookup value from a lookup field using Powershell in SharePoint 2010.
Web = Get-SPWeb "http://yoursite"
$List = $Web.Lists["Listname"]
$Item = $List.Items[0];
$Lookup = new-object Microsoft.SharePoint.SPFieldLookupValue($Item["YourField"])
$User = $Lookup.LookupValue;