At the top of the page is a large table, 21 columns long, by 6 rows. This represents the fretboard. Along the far left I've shown which Guitar String each row represents (E,A,D,G,B,E - from the bottom to the top).
To use this, simply run your mouse over the table. As you move through each cell, the cell should highlight to yellow.
To add a note to your tab, click on the cell that has the note you wish to play. You will see your note print out in the text box in the bottom.
When you are done creating your notes, you can cut and paste this into a tab site, or for your own personal use.