I thought Macs come with Git pre-installed but that seems to be the case only for recent models. You can check if your system have Git by opening Terminal and type in:
which git
It should print something back if Git is installed. If nothing happens, try downloading and installing git at http://git-scm.com/downloads.
After installing, make sure to quit and restart your Terminal. Then try which git again. It should be there.
The following assumes you already have git installed
Open up Terminal and enter:
git clone git://github.com/yyx990803/creative-html5.git
This should create a folder named creative-html5 inside your home folder (The folder with your username in Finder). Now do:
cd creative-html5
You should see the prompt now has creative-html5 in it, which means you are in that folder now. Now you can do
git pull
You should see it say that it's already up-to-date. That means things are working correctly.
Next time I pushed some updates to the repo, you just need to do git pull again. However, make sure you are doing that inside the creative-html5 folder! You can move the folder to somewhere else or rename it ( given that you know how to go to different folders with the cd command ), but don't put your own files in it or modify it.
If you run into problems, you can open an issue on GitHub.