I downloaded and installed 3D Desktop. What do I do from here? There isn't an option for it in the KMenu, and when I type in "3ddesktop" or "3d desktop" in Run, it can't find it.

Yes, I have my nVidia drivers installed and 3d enabled.

First get the daemon to acquire your desktop images:

$ 3ddeskd --acquire

If it is your first time running, then it will do this automatically

now:

$ 3ddesk

will activate it, and allow you to change desktops with your arrow keys, and choose a desktop with the space bar.

Now obviously you don't want to have to type "3ddesk" everytime, so either add it to your menu manually, or else do what I do: bind it to an unused key. I bound mine yo F9 key. How you do this is dependant on your desktop, but with KDE check out the khotkeys module in the control center.