StresStimulus can capture traffic from a mobile app. All mobile devices which allow you to set a proxy, such as an iPhone/iPad, Android, Windows phone or Blackberry are supported. The mobile device has to be connected through Wi-Fi to the same network that the StresStimulus machine is connected. In StresStimulus select Mobile from the record list. On the mobile device, set the network proxy settings to point to the StresStimulus machine's IP and set the port to 8888 if using Fiddler add-on or 49386 if using the standalone version (these are both the default ports and can be changed).
For more information on configuring Fiddler to capture traffic from another machine, check this source. Start navigating through a test scenario on the mobile device. When recording is finished, click Stop in the Recorder or in StresStimulus. A new test case will be created.