If you want to insert a page break, or a teaser break using tinyMCE view the little explaination here. Although you can use the Drupal Break plugin with older versions of TinyMCE, the best way to get it working with TinyMCE 3 is to use the pagebreak plugin that already exists and configure it to output the <--break--> tag that tells drupal where to split your teaser and page content.

http://drupal.org/node/247163