Recording "Stereo Mix" Output Using StarTech 7.1 Channel USB External Sound Card

I am trying to set up Audacity to record the audio of an internet radio station. I can not figure out how to enable the “Stereo Mix” for the External Sound Card. The “Stereo Mix” for the Realtek Audio is enabled but it will not record in Audacity when the External Sound Card is plugged in. Any suggestions?