adding limiter to reduce distortion in audio amp
You do realize, I hope, that the problem is that you're overdriving the inputs, right? and that a diode network will not prevent that, just make the results a little less nasty? And that the real solution is not to ask for too much output from the board?

OK, so you want to do it anyway. I hope you've got good eyesight and a steady hand. 

Looking at the PCB screenshot, the input connector is the 3-pin connector at the upper left. The signals are carried on the upper and lower pins, while the center pin is ground. You can see the two signal traces going right and down to your 2 capacitors just under the 5-pin IC. The right-hand pad of each capacitor is where you want to make your connection.

Good luck.