i don’t think that you have only + and -, you need GND
If you have made the modifications, as you said above, to power the Fart Box with a Eurorack power supply, you have 3 points (+/GND/-)
without the modifications the FB seams to work with AC/AC power supply ( only 2 points)
on Sam web site :
“as for the 12v AC AC power any will work! it needs to be AC AC not AC DC”
and the mixer module need a +12V/GND/-12V DC
maybe some infos about power supply for modules here