It sounds like you may be going to a link to "shared responses" instead of opening the actual form.
Sign into the FormsCentral account that you used to author the form, locate the form in the list of forms and open it up. You can make changes on the design tab. If all that you see when you open the form is a View Responses tab then that form was shared with you at the "Reader" level and you don't have permission to edit, you would want to either contact the author, or possibly locate a different version of the form that you own. Maybe you shared with yourself and need to sign into a different account to see the original form.