I had no luck using the Spindle PWM circuit to drive a SS Relay, I think (on mine at least) the SSR load isn't enough and the voltage stays high (~24VDC) on the circuit under so little load (~3mA). I ended up just using the Arduino output (the yellow and black wire) directly to activate the SSR and its been working great (for last several months). My SSR is 40A and the trigger is 3-30VDC something like that. I use UGS and just have to set the RPM over 12500 to get the arduino to do the full 5v out. I think there is a setting somewhere you can change to make it pure on or off, but its easy enough as is, works with Easel too.
you can confirm the arduino output is working using a multi meter should see 0-5VDC, spindle off or on. the test setup on easel turns the spindle on,... off,... on,... off....
kind of hard to see in the pic but I just used the phoenix plug as a terminal to join a shielded extension wire to the stock black and yellow spindle out wires.
sorry so long winded
shop vac and router plugged into the relay controlled outlet, the right side. the left outlets are always on, I use that for the laptop PSU.