Ubuntu comes with it already installed, all you need to do is enable the desktop effects
Open the compiz config and enable the cube and any other affects you want, you will need the 3rd part drivers enabled (i think ubuntu tells you this anyhow)
Looking at that error message would suggest your not using hardware 3d
OK. I can't remember exactly, as I'm on my work PC at the moment and therefore running Windows, but the process to get this going should be pretty simple. The first thing that you need to do is to enable restricted drivers for your graphics cards; I'm not entirely sure where this is in the menus but it shouldn't require the terminal.
Next up, you'll want to install ccsm (short for "CompizConfig Settings Manager"). I'm not sure if this is already installed on 9.10, but if not it should be easy enough to find in Synaptic.
Finally, go to the appearance menu, and enable effects. You can then run ccsm to change which effects are being used.
I think that it's just control and then left click and drag to use the cube.
What do you mean when you say that you can flip your desktop? Do you mean that when you drag the mouse you "pull" the desktop, and it flips over to a second one? How many virtual desktops do you have? If it's only two, you won't get a cube; you need four...