Using Charles from an iPhone
To use Charles as your HTTP proxy on your iPhone you must manually configure the HTTP Proxy settings on your WiFi network in your iPhone's Settings.
Go to the Settings app, tap Wi-Fi, find the network you are connected to and then tap the blue disclosure arrow to configure the network. Scroll down to the HTTP Proxy setting, tap Manual. Enter the IP address of your computer running Charles in the Server field, and the port Charles is running on in the Port field (usually 8888). Leave Authentication set to Off.
All of your web traffic from your iPhone will now be sent via Charles. You should see a prompt in Charles when you first make a connection from the iPhone, asking you to allow the traffic.
Remember to disable the HTTP Proxy in your Settings when you stop using Charles, otherwise you'll get confusing network failures in your applications!