If i create a newsletter or a landing page how can put it in the body of my email so my clients can i see?
Are you really mean here works with InDesign?
InDesign is not really the correct tool for that.
The kind of email you describe is a specialized item that requires hosted HTML, best attained and deployed by enlisting a dedicated service like Constant Contact or MailChimp. You can't effectively send such email messages using only your email client.
Does MailChimp or Constant Contact provide an interface where you can design the newsletter? Or is the design created using other adobe products and then imported into these campaign managers?
They provide plenty of templates or you can start from a blank screen and build it to your liking.
You can cut and paste HTML code into Mailchimp templates, so you could design using Dreamweaver tables. InDesign lets you export HTML, but the code is CSS styled DIVs, and not all email clients support DIVs.