Digital signature field option is not available in FormsCentral because it is possible to create Web Forms and PDF Forms with FormsCentral and it is possible to use signature fields only in PDF files.
If you have a PDF form created in Acrobat, or created in FormsCentral and saved as PDF form, if you open it in Acrobat XI, go to menu View -> Tools -> Forms and click on Edit you will have the option to add a Digital Signature field: