Note that Java and JavaScript are not the same. You need the JavaScript options, which will be different from the Java options.

Firefox (Mac):

Go to "Firefox" in the main menu at the top of the screen, then "Preferences". Under the "Content" tab, tick the box to turn on JavaScript, and click "OK". Then reload the current page.

Firefox (Windows):

Go to "Tools" near the top of the Firefox window , then "Options". Under the "Content" tab, tick the box to turn on JavaScript, and click "OK". Then reload the current page.

Internet Explorer (Windows):

If a box appears near the top of the screen saying "To help your security, Internet Explorer has restricted this webpage [etc]. Click here for options...", click in the box and select "Allow Blocked Content". Click "Yes" in the panel that then appears.

Safari (Mac):

Go to "Safari" in the main menu at the top of the screen. Then go to "Preferences", then the "Security" tab, and tick the box to enable JavaScript. Close the "Security" window. Then restart the browser.

Safari (Windows):

Go to the tools menu (the gearwheel at top right). Then go to "Preferences", then the "Security" tab, and tick the box to enable JavaScript. Close the "Security" window. Then restart the browser.

Other browsers:

Look under "Options" or "Preferences" or "Tools", then under something like "Security" or "Content". After you have found the option to enable Javascript, restart the browser. (Or try the browser's help menu!)