The 'planet' program which acts the engine for World Builder is command-line program developed by Torben Mogenson. If you want to play with the original program, you will find it in the 'planet' sub-directory underneath the folder where World Builder was installed. You should probably copy it to a different location before doing anything with it, though, because if you delete or change anything in this folder, World Builder will probably no longer work!
The manual for 'planet' itself, manual.txt, will also be found in the 'planet' folder.
The version of 'planet' used by World Builder is not completely identical to the one that can be downloaded Torben's website, the main difference being that World Builder uses a 32-bit compilation of it, instead of the original 16-bit version.
Most of the options available in 'planet' are also available in World Builder, the only major omission being the 'search' mode -- this has been replaced by the Explorer in World Builder, which performs a somewhat similar function, but in a completely different way.