Sign into FormsCentral, select the form and double click on the form. Your form will open and you are in editing mode
If you have imported a PDF form (say an old Acroform or static XFA form) into FormsCentral, then you will not be able to edit the form in FormsCentral. You'll have to use the original Adobe application that you used to create the form, and you'll have to re-import your form to FormsCentral.
Hope this answers your question,