The Circuit applet should load in it's own window. Try clicking on one of the prebuilt circuits (adder or xor).
You may select individual elements (by clicking on them), and use any of the action buttons (Depth, TruthTable, etc.). Unselect by clicking again.
You may change the value of an input (black or white, labelled with an "I") by clicking on it and selecting the corresponding value.
This applet requires JDK 1.1, so Netscape 4.06 or higher.