You can not use using namespace syntax until v5.0

OldStyleCallingDotNet.ps1

#Version under 5.0

function Main
{
    [OutputType([System.Collections.Generic.List[string]])]
    [CmdletBinding()]
    param()

    $listInt = New-Object System.Collections.Generic.List[int];
    $listString = New-Object System.Collections.Generic.List[string];
    [System.Linq.Enumerable]::Range(0, 10) | %{ $listInt.Add($_); }
    $listInt | % { $listString.Add("Item : $_"); }
    return $listString;
}

Main;