You may need to make Java 6 the default (top-most) option and have it enabled, as above.
To change Java preferences on Mac OS got to Applications => Utilities => Java Preferences
The Mac OS package download is very simple - it's just a ZIP file. To "install", all you need to do is download the ZIP file, decompress it and put the resulting application wherever you want. To "de-install", just delete it. Mac OS X comes with Java as standard and has a simple way to create Mac OS X style applications from Java code.
The Web Start version will only work if you have Java already installed and it configured within your browser. When you install it for the first time, you will go through a security check (giving the X3F Tool the rights to read and write to files and more), though it doesn't give it anything the stand-alone application cannot do. Every time you run it later, it will automatically check for updates and install them if available, which is part of the Web Start framework.
• On Windows go to Start => Control Panel => Java. Then click on the View... button in the Temporary Internet Files section and you will get a list of all Web Start applications. You can select and delete them there.
• On Mac OS go to Applications => Utilities => Java Preferences. Then go to Network => View Cache Files... and you will get a list of all Web Start applications. You can select and delete them there.
You may see this dialog - by default X3F Tool becomes the default program when you double-click on an X3F file. You can revert to another program in the normal way for your OS or by deleting X3F Tool.