The confusion is understandable.
FormsCentral is a subscription based service and is seperate from Adobe Acrobat. Acrobat Professional XI does come with a desktop application made by FormsCentral which allows users to create unlimited PDF forms, but use of online forms services is connected to signing in and which subscription level you have.
This FAQ talks more about the difference: http://forums.adobe.com/docs/DOC-1385
This comparison page shows you what you get with the Free and each Paid subscription levels with Acrobat Pro XI: https://www.acrobat.com/formscentral/en/landing/axifcpricing.html?promoid=KEURI&accntlvl=b sc
PS - there is an interesting subscriptionoption "Acrobat Plus" that includes a desktop installation of Adobe Acrobat Pro XI as well as subscriptions to online services such as FormsCentral, PDF Pack and ExportPDF: https://new.acrobat.com/en_us/products/acrobat-plus.html?s_tnt=67837:1:0
I found this same thing to be true! Very frustrating to have it for the trial period, buy it, only to find out it is an additional cost. I listened to a short video three times to make sure I heard him correctly. The video says it is included in the XI Pro version. It is very misleading.