FormsCentral doesn't support s-signature currently and there is no signature field.
Depending on your requirements for a "signature", some people will just use a checkbox that indicates that you are "signing" the form, maybe some text for the agreement and a checkbox to agree. I don't know if your work flow requires an actual signature, but wanted to give you that bit of information.
Open your PDF with Acrobat Pro to edit form fields (not document Content). There you have the still existing possibility to insert / edit signature fields in PDF forms.
To insert / edit sigfields into LiveCycle forms by Adobe you need the Adobe LiveCycle Designer.