+https://github.com/cakephp/cakephp/downloads (look for the 1.2.5 tag)
Expand the framework and put it in the $BASE directory. Delete the app directory that came with cake, and rename the directory containing the backchan.nl release you downloaded to "app" and move it into the $BASE directory. When you're finished, your $BASE directory should look like:
Ideally, when you can browse to your server and a backchan.nl-looking page shows up, you're making progress. There will almost certainly be errors (like "Error: The requested address '/' was not found on this server."), since you haven't setup the database yet.
+You might also need to enable the php5 module; it is not enabled by default on a fresh OS X 10.6 install. Just uncomment the LoadModule line for php5.