electronics setting up thc up/down/ok with relay outputs

I don't see why yoiu are worried. pins 5 & 6 can accept 24 volts with  a max of 36 volts why would the isolated inputs accept less?
Its going to depend if you are using active high or active low inputs as to what you connect to common So using a mesa card pin 5 should be - field power and pin 6 should be + field power
the outputs should be connected to Mesa inputs.
Common should be  + field power for active high or - field power for active low inputs (like planet cnc)
What makes you think it can't handle 6-36 volts?

Tried 24v, the relay lets 24 v pass through all the time.
Will get a 5v relay board and try to do it
Any suggestions on how to do it using a resistance ?

I checked the cpc1002n 732t4309 relay and it seems to hold 60v...
Should i have wired it some other way? I wired field 24v+ to thc com and ok/up/down to mesa inputs 1,2,3 , and field v- to mesa input gnd. All 3 inputs light up just by powering the circuit on and dont change while testing up/down. Will try with 5v tomorrow

i ended up getting a thcad and using a divider and it works almost 100%. im just having trouble getting arc ok to trigger. using mode 0.

selling a planetcnc thc xD

Using mode 0 is dependent on voltage thresholds to synthesise ans ArcOK signal.
I personally think this is a bad idea as it often does not operate as intended due to user config error or vagaries with the plasma cutter.
If your machine has an arcOK signal, use it. 
If not, there is a method described in the QTplasmac docs that uses a reed relay to create a physical arcok signal.