To start playing, click on the Run button. You will see a block fall immediately on the left of the grid. To add to that block, select a fraction on the right side by clicking on it. You should then see a block fall into place. Keep adding blocks until you have filled the row. If you fill your row up without using any fraction more than once, it will disappear so you can start a new row. Otherwise, your row will remain and you will start on the next row up. You'll have 250 seconds to finish as many rows as possible!
To play the game again, press the Reset button, then press the Run button.