ALT="Your browser understands the <APPLET> tag but isn't running the applet, for some reason." Your browser is ignoring the <APPLET> tag!
Each column, each row and each of the 9 3x3 fields must contain all values from 1 to 9. Black fields cannot be changed, red fields are incorrect.
Click on a field to mark it and then either click on the value button on the left hand side or use the numbers on your keyboard (not numpad) to set the cell-value.