William,
If I understand your post correctly you are attempting to wire up analog input channels 0 to 15 to take differential readings. To wire the device up in differential mode, the analog input channels of your device are paired together. Channels 0 through 7 are paired with channels 8 through 15 to give you 8 differential channels. So to wire a signal into differential channel 0, you would wire signal(+) to pin 68 (ai0) and signal(-) to pin 34(ai8). Channel 1 is paired with channel 9 and so on.
If you need to use more than 8 channels (which from your post I got the idea that you did) you will not be able to use differential mode measurements with this device. If all the signals you are measuring share common ground, you could use Referenced Single Ended mode, in which each signal is measured against AI GND or Non-Referenced Single Ended Mode in which each signal is measured against AI SENSE.
Hope this helps,
Dan