For Firefox, this is under "Options" item of the "Tools" dropdown menu. Click on the "Web Features" icon on the left, then check the box labeled "Enable JavaScript", then click on the "OK" button.
For Netscape and Mozilla, this is under the "Preferences" item of the "Edit" dropdown menu. On the "Advanced" item, go to "Scripts & Plugins" then click the checkbox for "Navigator" under "Enable JavaScript for". Also click all the boxes under "Allow scripts to:".
For Internet Explorer, this is under "Internet Options" item of the "Tools" dropdown menu. Click on the "Security Settings" tab, then the "Custom Level" button. Choose "Enable" in all the boxes under the scripting heading.