This is a java applet, which you need java to use. To download, go to http://java.sun.com/javase/downloads/index.jsp and download the latest 'Java Runtime Environment (JRE)', you do not need Java EE, Java SE, or JDK.
For preflop computations, the overall percentage is the mean share of the pot that the pocket cards will win in a 10 player ring game with no betting or folding. The simulator runs by dealing 9 opponents and 5 board cards at random, finding the winner(s), and repeating.
Postflop: Unfortunately, the overall percentage has less meaning for posflop computations, because this program is not taking in factors like betting history and position. The simulator works by dealing 9 random opponents and removing the worst preflop hands using the rankings in this table, the number of folders is determined by: numFolders = 9 - remainingOpponents. The simulator then deals the five board cards, finds the winner(s) among remaining opponents, and repeats.
I am currently working on some poker ai, and there will be a signed applet / software comming out at some point that will read the cards / betting history into the program for you to give more advanced evaluations.