Hi ACC768000 - we've moved this post to our SlingPlayer for Boxee Box forum area.
This page outlines exactly how to find and install the app on your Internet-connected Boxee Box:
And yes - SlingPlayer for Boxee Box is free! Note that you need a Slingbox PRO-HD or SOLO, a router, and an HDTV connected to a Boxee Box by D-Link.
Hope that helps!