Problem with adding Custom Reference to customization

I am trying to send an e-mail via customization using a System.Net.Mail namespace.
But
Imports System.Net.Mail
gives me the following error:
Namespace or type 'Mail' for the Imports 'System.Net.Mail' cannot be found.

So I was trying to use a Assembly Reference Manager to add a Custom Reference (System.Web.dll). But after selecting the file and clicking ok, nothing happend. The dialog box for selecting dll is closing and there is NOTHING in Custom Assemblies Tree.

However I can add without the problems the mfg*.dll.

How can I add a needed reference?