If you are using the FormsCenral website to create a form it will have a submit button by default for the HTML (web) form and also by default it you are saving as PDF (there is an option to create a PDF without Submit, but by default it does submit to FormsCentral).
If you are using the FormsCentral desktop application that is bundled with Acrobat XI you would have to sign into the service to create forms with Submit buttons. You can also create a form without submit button in FormsCentral Desktop and then edit the form in Acrobat XI adding a submit button using your preferred submit method such as by email.
Let me know if you need more information on either of these, it will be easier to assist knowing more about what you are using.
PS - if this is a question about adding a submit button in Acrobat not related to FormsCentral your question would be best asked in http://forums.adobe.com/community/acrobat