This page summarizes all available downloads, including links to where source code can be found.
LispKit is the Scheme interpreter used by LispPad. The source code of LispKit and the included libraries has been published at GitHub and can be downloaded from there. Example code and the sources of the included libraries can be found here.
The latest LispPad release can always be downloaded from the Mac App Store. Alternatively, this website keeps an archive of all releases with links to the corresponding binaries. The latest binary is available for download here.