RTD 100 hooked up to Lee Dickens BD-300 Isolating Signal Converter, Issues Reading the Output
If you were expecting a 1-5V output and you accidentally purchased a current output model isolator, simply connect a high-precision 250 ohm resistor across the output terminals (or even better, across the input terminals of whatever is connected to your signal isolator). Resistors of this type are commonly available for exactly this purpose.

Ohm's law:

V = I * R

V = [4mA - 20mA] * (250 ohms)

V = 1V - 5V

While it may seem like a pain, a 4-20mA signal is preferable to a voltage output in many applications:  


It allows the detection of a broken sensor wire (it goes to 0mA) in addition to sensing the process variable (sensor output).
Some sensors can be 'loop powered' - they require less than 4mA to operate, and can therefore be used as a two-wire device without need for a separate power supply wire.  The savings in wiring cost alone can be significant.
A current signal eliminates error due to resistive losses in the conductors between sensor and readout/ADC. (Though it trades this for errors related to the 250 ohm resistor)
Noise immunity is substantially better due to the inherent common mode rejection.  This is a huge win in industrial settings when tend to be very electrically noisy.