The Picmas Tree 1.1 is programmed in MPASM assembler using the MPLAB IDE which you can download from Microchip for free. Once you have installed MPLAB, you can download the source code for the tree and then open it up and build it. You will need to have a programmer to load it into the pic. We also have the .hex file which you can download.
We've documented the code extensively inline and in a complete multi-page flow chart which you can download here.