If you get an error EXACTLY as outlined in the image below, this is a known issue with pages.js
I have attached a pages.js file which will need to replace the pages.js file on your computer.
Make sure Dreamweaver is closed and browse to this location:
C:\Users\<Your UserName>\AppData\Roaming\Adobe\Dreamweaver CS4\en_US\Configuration\Shared\WebAssist\eCart\ChWizJS
backup existing pages.js first just in case
And then replace the pages.js file with the attached file.
Start Dreamweaver and run through the checkout wizard.