Check your version of Mac OS X. » How do I find my Mac OS X version?
Use Software Update available on the Apple menu to check that you have the most up-to-date version of Java 6 for your Mac. Java 7 is not supported by these older versions of Mac OS X. If you have problems with Java 6, contact Apple Technical Support. Java.com does not provide a download for these systems.
Java 7 can run on your Mac. Java 7 requires an Intel-based Mac running Mac OS X 10.7.3 (Lion) or later and administrator privileges for installation.
» Information about installing and using Java 7 on Mac