You can insert any variable in a text container (use that word to point to both text captions and shapes with text) using the small X button in the Format accordion of the text container when you are in Edit mode.
Maybe this introduction to variables could help you: http://blog.lilybiri.com/curious-about-variables-in-captivate-4-5 (still valid for 6)
And if you want to use the TextArea widget:
http://blog.lilybiri.com/widgets-and-custom-questions-part-2
http://blog.lilybiri.com/extended-textarea-widget-more-functionality
Lilybiri