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If you haven't installed Java yet, download and install it >http://support.apple.com/kb/DL1572 Keep Safari closed during installation and restart your computer when finished. It's not necessary, but it's to make sure that Java installation went properly.
Then, go to a Java site and see if you can watch Java content. It's important to keep it updated, and disable it when you aren't using it. You can do it in Safari menu (on the menu bar) > Preferences > Security