Using an OEM power supply
If your PSU looks like this 12 Volt (single voltage output), 30 amp power supply:




The adj label probably refers to an adjustment screw, not to any of the terminals.
The -V label does not mean -12V, it is equivalent to COM on the PSU in your reprap link. 
I would not expect the ⏚ terminal to have a low resistance to the +V terminal with all cables disconnected.