Sorry, we don't support a checkbox+text-entry field. "Others" option is only available within choice fields.
Thank you for your response Lucia. Can this suggestion be added to a "requested features" list for future consideration?
Yes. you can add this feature request to http://forums.adobe.com/community/formscentral?view=idea
If you need to add a new idea click "Create an idea" under "Actions" in the top right.