Use the Add method of Attachments collection as in following code:
// create a mail message and add attachment to it
MailMessage message = new MailMessage()
message.Attachments.Add(new Attachment("c:\\data.zip"));
Following code will fill values for some basic properties and will upload the message to the server via IMAP:
message.From        = "from@example.org";
message.To          = "to@example.org";
message.Subject     = "Mail subject";
Imap client = new Imap();
client.Connect("imap.example.org");
client.Login("username", "password");    
// upload the message into Inbox  
client.StoreMessage("Inbox", mail);
You may also find following tutorials handy: