The -5V line has been totally deleted from the ATX spec as of the last rev. It has been optonal since ATX 2.0. As others have said above, nothing uses the -5V line any more so fageddaboudit...
. Switching regulators have a problem regulating without a load (nothing uses = no load) - so if you want that to show an accurate reading, tack a 100 Ohm resistor between the -5V line and ground.
. The polarity of Voltage is expressed relative to Ground (0 V) so +5V means 5 volts above Ground and -5V means 5 volts below ground. If you connect the +5 to one end of a resistor and the -5 line to the other end, then you have 10 Volts across the resistor.
.bh.