The most current version as of this writing (0.7.5) lets you open it using ("File | Open").
Use "File | Open", navigate to the AdaFruit.fzbz file and open it. The library should import automatically, and you'll see as it populates the parts palette on the right-hand side with all the new Adafruit parts.
The next thing you want to do is save the Adafruit bin (parts libraries in Fritzing are called 'bins'). Do this with the 'Save Bin' command in the parts palette (as shown in the image above). By default, Fritzing will want to save the bin to somewhere in your "/user' directory, which is the best place for it, so approve the suggestion and save the bin.
Restart Fritzing (close it and run it again). Make sure you've saved the bin before restarting or it won't show up in the next session.
You may now delete the copy of "AdaFruit.fzbz" from wherever you put it when you extracted it from the GitHub archive.