All information concerning the development and beta testing of Pingus is at a separate section on this webpage (which is currently out of order), so that players don't accidentally see stuff that wasn't yet meant for release or grab a development snapshot, instead of a stable binary by accident. So if you are a player don't enter this section, but instead stay on the light site of the Pingus webpage and simply wait for the next stable release which will contain everything in a nice packaged and useable form, going to the development section will just spoil you a lot.
If you are or want to become a developer you are of course welcome to the dark site of Pingus, there you will find source code documentation, a todo with stuff that needs to be done and unfinished game elements that might need you to finish them.
Short summary for those who just want the source:
git clone https://github.com/Pingus/pingus.git