Download the current Java SDK including NetBeans from Oracle's Java page and install it. Test now whether the Java plugin for your browser is installed by looking at an applet page, e.g. the mathematical pendulum. If necessary download and install the plugin.
After the installation you will find the directories for the user-specific part of the NetBeans configuration at /Users/USERNAME/Library/Application Support/NetBeans/7.2/config.
Finally create an arbitrary directory for all your PhysBeans activities, e.g. /Users/USERNAME/physbeans, and copy the latest PhysBeans library physbeans.jar from the download page into it.
Now proceed with the configuration of NetBeans.