It is unfortunately not possible to insert a Flash, into a newsletter. Not many e-mail clients supports these formats since they are protecting the users from XSS (Cross-Site Scripting Attacks), and they have an adverse effect towards spam-filters.
If you wish to “insert” a flash, you can instead use a background image in the table cell or DIV the flash movie is being placed.
This article shows some extensive testing of Flash, and the problems that are associated on using it in email. In short, even they have determined, you cannot, and should not, use Flash in email.