After I installed the player, it said that it wasn't working but I went ahead and ran the widget and it did work. I did this on a Windows 7 64-bit machine with IE9.
I clicked on the three + signs to expand the controls which I think is very important to do. I found one minor problem. Whenever the x' value is zero, it displays an incorrectly formatted very small number with an exponent. For example, set t=5, x=3, v=0.6.
I thought this was only going to be a conversion of x and t coordinates from one frame to another moving at some v and it does that but in addition, it makes a Minkowski diagram showing the two frames.