The source for the 3D viewer is in code.lisp. You need to compile code.lisp and then load it.
From the REPL type (start). Up pop two windows, one containing a square, the other containing a cross. Move the mouse onto the cross. When you get it on to the center of the cross, pixel precise, the cross will disappear and the square will start to rotate, revealing that it is real a cube. Press a mouse button to let got of the cube. The cross will reappear, so you can pick it up again in the same orientation.
The REPL is still active (the interaction macro, though crude, is ANSI CL and should work with every implementation). Try
(setf *lines* (make-icosahedron))When you pick up the cross again an icosahedron will appear. The code also contains functions for making a globe and a torus.
If you are learning CL it is fun to gain experience by writing code that draws to the screen. However, if you are learning CL and lack experience then drawing to the screen under X11 is likely to prove too difficult. This is offered as get-you-started code.
It is supposed to "just work". If it didn't work or if you needed to tinker to make it go, please let me know and I will try and fix it.