I was trying to do something similar. Consider this as an alternative. If you have an iOS device, you can run your app on the device and configure Charles to only show traffic from remote clients by disabling the "Mac OS X proxy"
Here is an example setup:
Using one remote client (say an iPad in this case) and a Mac running Charles as your proxy host, disable the "Mac OS X proxy" by deselecting the checkbox in Proxy Settings >> Mac OS X Tab >> Enable Mac OS X proxy (See image below.)
By doing so, you essentially filter your simple setup to only display traffic that is coming from remote client (the iPad).
