First off, there is already a password checker built into the OS. You can get to it via the Accounts System Preference when you choose "Change Password." However, that's not very convenient. You can get a program to open the assisstant here:
http://www.codepoetry.net/products/passwordassistant
If you still want John, you'll need to have the Developer Tools installed. If you didn't select that option when you installed, you will have to go into the optional installs and install it.
From the Link you posted:
You must download the archive and unarchive it to a folder. There are directions linked in the download page of the website. In Terminal, change to the source folder that is inside of that folder using cd (change directory).
cd src
--In this line, he assumes you have already cd'd into the unarchived folder. The easiest way would be to type cd and space into the terminal, and then drag the src folder to the Terminal window, then hit return.
make
Note the make target for your system and type:
--Apparently, make will tell you what you need to type in for target. I didn't download and try to install, so you'll have to sort that out yourself.
make clean SYSTEM
where SYSTEM is the appropriate make target. Alternatively, if your system is not listed, use:
make clean generic
If everything goes well, this will create the executables for John and its related utilities under "../run/". You can change directory to there and start John, like this:
--The make command creates an executable inside your unarchived folder inside /run.
--From where you are, change to the run directory
cd ../run
--Now run JtR by typing:
./john --test
Alternatively, you may copy the entire "run" directory to anywhere you like and use John from there.
This is not going to create a double-clickable gui app. You have to run it from Terminal or create an executable shell script to run it via double-click.