1. Open CMD on your PC (Start - Run - CMD) and type in this command:
ipconfig
2. Get your IPv4 address from the connection you have. I'm using an Ethernet cable so mine is this:
3. Go to your iPhone Settings -> Wi-Fi -> WIFINAME (click the round i icon) -> Tap on Manual for HTTP PROXY and enter the IPv4 address you got from step 2. Set your port to 8888 and leave authentication off.
4. Go back out of your Wi-Fi settings and make sure Charles is running on your PC.
5. Open MovieBox/PlayBoxHD and Charles will ask you if you want to allow connection from your iDevice, click yes.
6. Play a movie and Charles will display the URL of where the stream is coming from.
Remember to disable HTTP Proxy once you're done.