Are you using the right version of Java? Find out here.
If not, download Java here.
Chrome users, be aware that:
Chrome does not support Java 7. Java 7 runs only on 64-bit browsers and Chrome is a 32-bit browser.
If you download Java 7, you will not be able to run Java content in Chrome and will need to use a 64-bit browser (such as Safari or Firefox) to run Java content within a browser. Additionally, installing Java 7 will disable the ability to use Apple Java 6 on your system.
Mac users must download the most recently updated version of Java, version 7 update 13.