The checkout wizard was no a part of eCart 3, it was not introduced until eCart 4.
eCart 3 is also not compatible with Dreamweaver CS3, version 3.7.1was released on 11/9/2006, DWCS3 was not released until April 2007.
you will not be able to use eCart 3 in Dreamweeaver CS3. Please uninstall eCart 3.