Step 1: Download a MailMessage object using IMAP or POP3 protocols. Check out the IMAP tutorial or POP3 tutorial for sample code.
Step 2: For each Attachment object in MailMessage's Attachments collection, call the Save method to save it to a file. Check out the ExtractAttachments sample or its description for sample code.
If anything is unclear, don't hesitate to ask again.