I want to to use the TitlePrefix statement. But it doesn't work: I always get an error message "Command 'TitlePrefix' not available". What can I do?

Please install a new GuiXT version. The TitlePrefix command came with version 2001 Q4 05 in December 2001. By the way, you can always find a GuiXT version overview (enhancements and corrections) in http://www.synactive.com->Download->Version info.
On the bottom of this page, click on "All previous versions".