I just purchased a DC bench power supply from Ebay. I have been trying to power up an op-amp circuit but with no luck. I checked the voltage of each output terminal with reference to the ground(earth ground) on the power supply. There is the correct positive potential but the negative terminal is showing no difference in potential as compared to ground. I am correct in believing this should show the negative 15V I am looking for and not 0V correct?
Your power supply has three terminals but they're not positive, negative, and ground, as you think. The red terminal is positive, the black terminal is 0V, and the green terminal is connected to the mains ground. The difference between the green terminal and black terminal is that the black terminal is only connected to a transformer, not to mains ground.
Unless the supply has separately adjustable positive and negative voltages and currents, it probably is a unipolar supply with a separate earth ground, and can only supply a single voltage. For example, this one (randomly selected eBay listing):
If you want a second negative supply, you can buy another power supply and tie the (+) output of one to the (-) output of the other, and typically you'd connect the earth terminals together (usually the output floats with respect to earth ground).
You might want to tie the earth grounds to the common between the supplies (but not always).