Well, both error messages are self-explanatory: first one is telling you that it cannot find that constant that you are referring to (you need to qualify it with the class/interface name it is defined in). Second error message is telling you that the method is not static method, you need an object of type PdfStamper before you can call this method (remember stp object?)
IMHO, what it shows is that you need to get up to speed with java basics, copy pasting code from different sources is not going to take you very far. I am not attempting to be rude, you need to pick-up a good book on java and practice a little, it will help you figure out many things on your own.
This thread seems more suitable for OtherOpenSourceProjects forum.
subject: java program to password protect a signed pdf file