How do I add a checkbox or text box to a document?

These options (and others) can be found in the toolbar on the Signatures tab.

In the "Add Signatures" tab, select the party you wish to assign a checkbox or text box to. Drag the "signature" icon onto the document where you want the person to sign (text boxes and checkboxes must be associated with a signature). 

With the signature highlighted, you will now notice that the Text Box and Checkbox icons are now available. Drag the desired icon over to the form.