Internet Explorer: Choose Tools > Internet Options. On the 'Security' tab, click the 'Custom level' button. Scroll down to 'Active scripting' and select 'Enable'.
Firefox: Choose Tools > Options. On the 'Content' tab, select 'Enable JavaScript'.
Safari: Choose Safari > Preferences. On the 'Security' tab select 'Enable JavaScript'.
If you're using a different web browser, you can easily discover how to enable JavaScript by typing the name of your browser application plus 'enable JavaScript' into any search engine (such as Google).