If you are a paid author, and enabled payment feature, you should be able to filling out web form with payment feature.
If you’re a free user, there’s a upgrade button on payment tab. You should easy to upgrade to paid user to own a lot of features.
And also, you can copy a form link to your website under distribute tab. Or you can embed a form to your website, through copy embed code to your website code. The form functions should same with your web form. Please FYI more embed form details on the doc. http://forums.adobe.com/docs/DOC-1991