I'm still not sure I understand how the signal processor fits into the picture:
- Keyboard outputting to the signal processor's input?
- Or signal processor outputting to the keyboard's input?
If the former, you want to run an audio cable from the keyboard to the signal processor.
If the latter, I don't understand what you're trying to do.
I guess it would help if I knew exactly what model of signal processor you're talking about, so I can look it up online and see what its capabilities are, because right now I'm picturing it as an effects pedal that's used to process the audio from an electric guitar or other instrument before it goes to an amp. And if that's the case then I assume you want the keyboard's audio to go to the processor.
But if you're talking about some sort of device that can transmit MIDI messages, and you want to use it to trigger certain sounds from the keyboard, then you'll need to program the processor to send specific MIDI messages to the keyboard for triggering specific sounds.