Cick the blue CSS Form Builder: area at the top of your form to get the 'edit' boxes to show in the Properties Inspector/Panel.
- Click on 'Edit Design'
- Select 'Form Elements' from the select list
- Select 'Captcha' from the 'Elements group' select list
- Make any adjustments here - (Background colour, noise, size, font etc).
- Click 'Finish' when you have done.
You won't see the results in Dreamweaver design view, but they will show when previewed in browser.