You need to ensure that JavaScript is switched on.
How to do this in Internet Explorer:
From the Tools menu select Internet Options.
From the Security tab set the security level to Medium. JavaScript will automatically be enabled. If you wish to have more control over your security settings, select the Custom security level and click on Settings. Scroll down to the bottom of the settings window and ensure that Active scripting is enabled.
How to do this in Firefox:
From the Tools menu select Options.
From the Content tab make sure the Enable JavaScript checkbox is ticked.
How to do this in Safari:
From the Safari menu select Preferences.
From the Security tab make sure the Enable JavaScript checkbox is ticked.
You will need to look at your browser's help to see how to do this on other browsers.