I think that's a "Desktop Wall", not the cube. You can have several effects set up with Compiz to do one task (for a lot of tasks), so for example to change desktop (or view all) you can see them as a wall, as a cube (or cylinder, depending on how you prefer to have it served) and so on. You can even have, depending on the case at hand, multiple such things enabled simultaneously and use them with different shortcuts (keys or mouse). So if you have enabled the cube and happen to see a wall, it doesn't mean the wall would be the "cube" -- you're just calling for the wall, not the cube.
You should familiarize yourself with the compizconfig settings and what you have enabled and what not. Disable those you don't use, enable those you do use, and if there are actions that are handled by multiple things, you're asked which thing you want to handle what, and answer accordingly.
Like mentioned, you would probably want something like 4 desktops for the cube. Two will show up a plane with two sides (obviously, sort of a "2-sided cube"), four resuls in a cube with four walls as desktops and two as logos (images), and so on. Tweak the settings and the cubic object turns into a cylinder with desktops surrounding it. So, open the configuration tool and read before you buy.
The shortcuts for using each Compiz thing are also presented in the appropriate configuration windows (don't just "enable" them, also see their settings), so if you don't know how to use the cube, see the key combinations there. Typically you hold ctrl and alt, then drag the desktop with your mouse to zoom out to the cube; with ctrl+alt+left/right/down you may end up using the wall or the cube, depending what choices you made when you enabled the things.
Last edited by b0uncer; 10-18-2009 at 01:42 PM.