Yes, the trial version has the same functionality as the fully licensed version - the only difference is that you can only use it for 30 days.
Is it possible that your document's security settings don't allow modifications? You can check by bringing up the document properties (Ctrl-D or Cmd-D), then go to the Security tab. Are there any restrictions listed?
Karl Heinz Kremer
PDF Acrobatics Without a Net
PDF Software Development, Training and More...
http://www.khkonsulting.com