ISSUE:
I created a Salesforce Community, tried to edit the community using Site.com Studio and could not find a way to add DATA and form elements. This is how my editor looked like.


RESOLUTION:
Change the template to 'Salesforce TABS + Visualforce' and the DATA and FORM elements will be available. The DATA and FORM elements are not available/visible with the Kokua, Koa, Napili and Aloha templates.

If you have already created pages with any of these templates, you will have to delete ALL pages and then change the template to 'Salesforce TABS + Visualforce'. Any new page you create after this will have the DATA and FORM page elements.

Salesforce Official Help link: Click here