The Actor button, Where is it?

Hello
I am trying to learn how to use the built-in physics engine, but I couldn’t find this Actor button in my interface!

http://o9o9.net/2/uploads1/22f35a3544.gif

Here is the panel as it appears in my blender:

http://o9o9.net/2/uploads1/bfd1365f82.gif

I have the 2.42a version, do I have to install something in order to have this option?
thank you.

.

Use your MMB to pan that window down; the button is hidden under the header.

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