Okay, so measure between pins with the meter set to ohms and look for something in the 10-40 ohm range, I would expect. That will be the motor on your spool gun.
When I'm measuring connectors like this, I'll use the following pattern:
1-2
1-3
1-4
1-5
1-6
2-3
2-4
2-5
2-6
3-4
3-5
3-6
4-5
4-6
5-6
That way you won't miss any combinations.
Mark down which pins those are.
Then you'll need to find two pins that connect when you pull the trigger switch, on both guns.
Once you find the pins for the gun that comes with the welder, you can short out those two pins on the welder itself. Then measure around for the 24V or whatever drives the motor, and connect those to the pins that connect to the spool gun motor. If you get it backwards the motor will run in reverse.
That should be about all you need to do.