You have added the using statements, but if you have not added a reference to the actual DLL file in your references then you will get this error. You will also get this error if you are using a version of the DLL that does not include this method. Here is a Code Project article from 2008 that uses your code:
Programmatically Complete PDF Form Fields using Visual Basic and the iTextSharp DLL
If the above two suggestions don't get you far enough, I would recommend downloading the source from this article and trying to run it. If it runs, compare it to your code to see what the difference is. Most likely it is either the missing DLL or the wrong version of the DLL.