Type the crib into the "Plain text" field and enciphered data into the Cipher text field of the applet and press the Start button. Alternatively use the Example button and then press Start. This should produce a drop of 123-CBA (AH) (LY) (NS) (OR). The full test takes about 1 hour and 45 minutes on my 100 Mhz Pentium using IE 3.0. The version I have in C (that can be downloaded from here) does the same thing in only 8 minutes.
The "Unknown ring setting" option represents the way the real Bombe worked. In this mode the crib being tested will not work if either the medium or slow rotors moved when the corresponding cipher text was produced. A good solution to this problem is to chop the crib into two halves. One of two halves should be free of middle and fast rotor movement.
The other option is just for fun. The rotors can move whenever and it will not matter, but all the rings must have been set to A.