BUT the links on your download page for Date Selector, Swing Extras, and Hexagonal Buttons all link to DateSelector.zip.
Thanks for letting me know. I've fixed it now.

http://www.keang.co.uk/downloads/SwingExtras.jar
Good guess but not quite correct. It's actually in a zip file as the code is there as well as the jar, so you need http://www.keang.co.uk/downloads/SwingExtras.zip. You can either include the jar in your project or just the VariableGridLayout.java file (in the uk\co\keang\swingaddons directory) and paste it into your project remembering to change the package statement.