The scenarios on this website are Java applets, but some have also been converted to HTML 5 (this is a "beta" feature at the moment). The Java version is displayed by default; to run it, you must have the Java Runtime Environment (JRE) installed. The JRE includes a web browser plugin that works with major browsers (Firefox and Internet Explorer; for Chrome, see this note). However, support for the Java plugin is being phased out of browsers.
If a HTML 5 version of a scenario is available, a link titled "HTML5 version" will be available under the Java applet (or under the text saying that the applet tag isn't supported, if you don't have a suitable browser plugin). If the Java applet doesn't run for you, you might wish to try the HTML 5 version by clicking the link.
Scenarios are written by members of the Greenfoot community, many of whom are still learning to code in Java, and some of them may not be functional. If some scenarios work for you but some don't, it's possible that the not-working scenarios are incomplete or have bugs. You can try posting a comment on the scenario to encourage the author to resolve the issue.
Finally, some scenarios include an "open in Greenfoot" link. If you have Greenfoot installed, you can use the "open in Greenfoot" link to download the scenario as a file that you can open using the Greenfoot application.
You can download the JRE (including browser plugin) from here:
Make sure to get the correct download for your operating system.
If you are using Microsoft Windows, you need to determine whether you have a 32-bit or 64-bit operating system.
If you have a 32-bit version of Windows, you should download the 32-bit Java environment which is labelled as Windows x86 and named something like jre-7u51-windows-i586.exe or jdk-7u51-windows-i586.exe.
If you have a 64-bit version of Windows, you can theoretically run either the 32-bit Java environment (labelled as Windows x86) or the 64-bit Java environment (labelled as Windows x64). However, you should choose depending on whether your browser is 32- or 64-bit. See this webpage for guidance.
If you are using a Linux distribution, you may be able to install the Oracle JDK via your distribution's package management system - check the distribution documentation. You can however still install the JDK manually be downloading it from the Oracle website (link above). You may also need to read the installation instructions.
If you see a message saying "Application Blocked by Security Settings", you need to configure the Java plugin to allow running applets on the greenfoot.org website. To do this:
Go to the "Security" tab.
Under the "Exception Site List" heading, click the "Edit Site List..." button.
Click "Add"; a dialog appears. Enter
http://www.greenfoot.org/, and click "Ok".
A warning appears saying that "Including an HTTP Location on the Exception Site List is considered a security risk." Click continue.
Click "Ok" to close the Java Control Panel.
You may now need to restart your web browser.
If you get the "Application Blocked" dialog when you try to run a scenario, with a complete error message reading:
Your security settiings have blocked an applicaiton from running with an out-of-date or expired version of Java.
... then your Java version if most likely out-of-date. You should install the latest available version.
Be aware that some scenarios may have bugs or problems which prevent them from running. You should try several scenarios before concluding that you are unable to run any scenario.
If you do have any other problems getting scenarios to run, try posting in the discussion forum.