How can I have my ISE read mV output instead of mg/L?

There is an experiment file (in /Probes & Sensors/ISE) in Logger Pro (post 3.4) that does that does this for you using the equation

23.92*(13.8-ln("Nitrate"))-168.23

This puts it in pre-amplification mV. If you want post-amplification Volts, the equation is:

V = 0.00727*mV + 1.223