You've received your Flex and you are excited to connect it up to your main speaker (L&R) and to your subwoofer. You didn't purchase miniDSP's TRS cable but instead used that TRS to RCA stereo cable lying around the house. It's most likely looking like this. 

You connect everything up and voila, no audio. What did you do wrong? 

Good news, everything on the Flex is indeed perfectly fine. Bad news you're using the wrong cable. :-) 

This specific TRS to RCA cable is a so called "Stereo Audio cable". It's meant to send stereo audio. The outputs of the Flex isn't a stereo output. It's a "balanced audio" signal where one signal is in phase, the other signal is out of phase. 

On a subwoofer, you'll typically see both Left&Right RCA input that are "summed" internally inside the plate amplifier of the subwoofer. 

If you connect up such cable, you're basically sending 2 x signals (out of phase of each others) that are mixed and perfectly cancelled. It's therefore 100% expected that you will not get any signal on the subwoofer. :-) 

OPTION1: Get a TRS to RCA cable as per miniDSP app note.

OPTION2: Just don't get BOTH RCA to the subwoofer. Just connect one side (E.g. Left). That will be fine since L&R are already mixed inside the Flex DSP matrix mixer. 

Hoping this info helps!