You are correct, the -12V line is the one you need to use. You have to change your thinking for negative regulators -- just remember you need a negative source to produce a negative output. That is why you cannot get -8V from +12V without special circuitry; -12V is the easiest available voltage on the PSU to derive -8V from.
The 7908 pinout is as follows (looking at the front, with the writing on it and pins facing down):
Pin 1: Ground (black wire on the PSU)
Pin 2: -12V input (blue wire on the PSU)
Pin 3: -8V output
The -12V wire on an ATX power supply should be blue in color. If you are looking directly into the ATX connector on the PSU, with the latching tab on the left, the -12V pin is the second pin from the bottom on the left column of contacts.
I hope this clears up some things. If it doesn't I can draw some diagrams to help you.