You are doing it right. The only thing is your server still showing the old text, it's because you have not removed the old jars. The only jar you must change in order to see the new text edited, is l2f-gameserver.jar.
Also you have to delete/remove the old jar from Libs folder... if it still showing the old text, is because you are renamed the old jars and keeping them at the same folder.
Again, after you compiled the full core, copy them and paste on you Lib folder server.
BUT, even editing those lines, you must know how donation system works, and I totally recommend to do not use it. I remember when we were using this system, and we had some troubles and it crashed, so it wasn't stable at all!! Also you need to setup your gmail in order to let the system read your emails.
Try to avoid setting up this and use other type of automatic donation system.
Cheers.