Install Mercurial for Mac. Grab the latest package from http://mercurial.berkwood.com/
For Linux, installing Mercurial depends on your Linux flavour. For example, on Ubuntu it's a matter or running "sudo apt-get install mercurial".
Open the terminal app and execute the following:
$ cd ~/
$ mkdir GoogleWebFonts
$ cd ~/GoogleWebFonts
The above commands will create a folder called "GoogleWebFonts" at the user's folder.
Let's grab the initial repo (do this only the first time you setup this whole process). Still in the terminal, execute the following:
$ hg clone https://googlefontdirectory.googlecode.com/hg/googlefontdirectory
This command will retrieve the entire Google Web Fonts repository (around 2GBs at the time of writing) inside this "GoogleWebFonts" folder we previously created. It may take from a few minutes to over an hour to download these files depending on network activity and your internet connection speed.
Now let's grab any font updates and zip everything to a handy, clean of irrelevant files, date-marked zip file. At the terminal app, execute the following:
$ cd ~/GoogleWebFonts/googlefontdirectory
$ hg pull --update
$ cd ..
$ find . -iname *.ttf -print | grep -v "src" | zip GoogleWebFonts_$(date +"%Y%m%d").zip -@
The above process will create a file called e.g. GoogleWebFonts_20120816.zip (if we run this process on Aug 16th 2012). The file generated at the time of writing this tutorial was around 70 MBs.
Now you can just drag and drop the fonts directly to the Mac "Font Book" app or to make it dead-simple (and cooler of course) extract this zip files inside your ~/Library/Fonts/ folder using this command (if the produced zip file was GoogleWebFonts_20120816.zip):$ mv ~/GoogleWebFonts/GoogleWebFonts_20120816.zip ~/Library/Fonts/
$ cd ~/Library/Fonts/
$ unzip GoogleWebFonts_20120816.zip