If you are using ssl for your site (and you probably should be) you'll need to install the charles certificate on your device.
Just navigate to http://www.charlesproxy.com/getssl and say yes and install to all the prompts.
Now in order to send the traffic from your phone to your Mac, you need two things, your phone and computer (hosting charles) need to be on the same network, probably wifi. You need to set you computer to act as a web proxy for your phone. You'll need your computer's IP address for this. The quickest way to get this is to Option-Click on the WiFi icon in the menu bar.
Follow these instructions to set the computer's ip address as your http proxy on the device