System.Windows.Forms reference is not recognized

Hi,

I have this code so that it can handle confirm dialogs:
Manager.DialogMonitor.AddDialog(AlertDialog.CreateAlertDialog(ActiveBrowser, DialogButton.OK));
Manager.DialogMonitor.Start();

But I got this error:

(CS0103) The name 'AlertDialog' does not exist in the current context
(CS0103) The name 'DialogButton' does not exist in the current

I added the System.Windows.Forms reference (following the instructions from this link):
http://docs.telerik.com/teststudio/features/coded-steps/add-assembly-reference

I also added this on top of the code behind file:
using System.Windows.Forms;

After adding the reference and the using statement, I still get the error message

Question:
1. What .dll should I use? I found 3 files with the same name(System.Windows.Forms.dll) but in different locations, I tried to reference them one by one but still the same error message appears
a. C:\Windows\Microsoft.NET\Framework64\v4.0.30319
b. C:\Windows\Microsoft.NET\Framework\v4.0.30319
c. C:\Windows\Microsoft.NET\assembly\GAC_MSIL\System.Windows.Forms\v4.0_4.0.0.0__b77a5c561934e089

2. Is there something that I am missing? how could I make this work?

Thanks!