FormsCentral does not include or support Signatures. For Signatures you can look at Adobe Acrobat solutions and Adobe EchoSign but they are not supported in FormsCentral.
After you have created the Basic PDF form in FormsCentral, you should save the form as PDF (form). Then open the saved PDF with Acrobat Pro XI for editing form fields (not editing for document content). There you have the possibility to insert / edit signature fields.
To insert / edit signature fields in LiveCycle Forms, you need the LCD (LiveCycle Designer). Since Acrobat XI you must buy the LiveCycle Designer as a seperate product by Adobe. Before the LCD came with Acrobat Pro.