Make sure you open it using the Adobe Reader Mobile. The iOS PDF viewer can't properly handle PDF forms.
I went to the ipad application store and there is NO Adobe Reader Mobile. Plus when I go to use the browser to fill out the form FormCentral tells me that it does not support the browser on the iPad.
Do you have the latest iOS version?
Are you trying to access the main FormsCentral application or the HTML form?
The Main application is Flash based so it won't work on the iPad. The HTML forms will - make sure you are doing to the URL provided in the Distribute tab of the application.