How to add existing user to exchange mailbox 2010

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.NET3.0

C#3.0

C#

.NET

I am trying to add existing user to he exchange mailbox 2010 in windows server 2008R2

I installed power shell in my XP system

and trying to access exchange mailbox through powershell from my c#.net Application

i am getting the following exception in the below code

The type initializer for 'System.Management.Automation.Remoting.Client.WSManClientSessionTransportManager' threw an exception.

SecureString password = new SecureString();
        string str_password = "123";
        string username = "xyz";

        string liveIdconnectionUri = "http://exchange.abc.com/Powershell?serializationLevel=Full";

        foreach (char x in str_password)
        {
            password.AppendChar(x);
        }

        PSCredential credential = new PSCredential(username, password);


        WSManConnectionInfo connectionInfo = new WSManConnectionInfo((new Uri(liveIdconnectionUri)), "http://schemas.microsoft.com/powershell/Microsoft.Exchange",
        credential);

        connectionInfo.AuthenticationMechanism = AuthenticationMechanism.Default;    

        Runspace runspace = System.Management.Automation.Runspaces.RunspaceFactory.CreateRunspace(connectionInfo);

        PowerShell powershell = PowerShell.Create();
        PSCommand command = new PSCommand();

        command.AddCommand("Enable-Mailbox");   
        command.AddParameter("Identity", "Peter England");
        command.AddParameter("Alias", "peter.england");
        command.AddParameter("Database", "MBX_SBG_01");

        powershell.Commands = command;
        try
        {

            runspace.Open();

            powershell.Runspace = runspace;          
            powershell.Invoke();
        }
        catch (Exception ex)
        {

            Console.WriteLine(ex.Message);
        }
        finally
        {

            runspace.Dispose();
            runspace = null;            
            powershell.Dispose();
            powershell = null;
        }

can anybody help