What version of CSS Form Builder are you using?
If all you are trying to make use of is the Form Toolkit Captch validation, you really don't need to install CSS form Builder along side Form Toolkit, all you really need to have installed is Form Toolkit for quickly adding captcha to the forms and adding validation.
Form Toolkit and Form builder are really for 2 completely different workflows:
Form Toolkit would be used to if you want a tool to create the forms for you.
form toolkit would be used if you prefer to create you own forms and add validation manually.