In order to work with the ObjectManager you will need:
You can download the latest Java Runtime Environment at: http://java.com/getjava
To start ObjectManager:
You can also use objectmanager.bat(Windows) or objectmanager.sh (Linux) if you have problems with the above-mentioned methods.
For further information on how to use ObjectManager, please see the documentation at: http://code.google.com/p/db4o-om/wiki/OM_Tutorial