I'm not entirely sure what you mean?
In most cases you should parse an email using MailBuilder class:
IMail email = new MailBuilder()
.CreateFromEml(imap.GetMessageByUID(uid));
then using IMail.Attachments (or IMail.Visuals, IMail.NonVisuals, IMail.Alternatives) collection is usually enough to access all attachments or visual elements.
To access MIME document directly you can use IMail.Document.Root property.