If you find that games on the web do not work for you at all -- for instance, if you see only a red X in the upper left corner of the area where the game should appear -- you probably do not have the Macromedia Flash player correctly installed. To fix this, download the Flash player from Macromedia's website and follow their installation instructions. A smaller number of games are written in Java. If you are still unable to play games after installing the Java and Flash plug-ins and restarting your computer, your problem may be a side effect of spyware and other invasive software on your computer.
Hope that Helps,
Justin