I know that it is confusing. But variables for widgets have always to be defined separately in the Variables dialog box. Have a look at this:
http://blog.lilybiri.com/using-captivate-widgets-some-tips
And I blogged about the TextArea widget, and also about an extended version by Jim Leichliter (it is free):
http://blog.lilybiri.com/widgets-and-custom-questions-part-2
http://blog.lilybiri.com/extended-textarea-widget-more-functionality
Perhaps I'm annoying with all those links, but that kind of information will not be found (easily) in the Help.
Lilybiri