Arduino analog input from an amp that uses a reference pin?
From the datasheet you can see that this reference pin provides the 0V reference for the output.  You can think of this as a level shifter feature.  The output voltage appears between the output and the reference, not between the output and some fixed voltage like ground.  If you want the output referenced to ground, you simply tie the reference pin to ground at the place the output voltage will be used.

If the differential signals you are measuring are always one polarity, then the reference pin should be tied to the analog ground reference of the microcontroller.  If the signals you are measuring are bipolar (the difference between the two input signals can be both positive and negative), then the output will swing both above and below the reference input.  In that case, you should tie the reference pin to 1/2 the microcontroller supply voltage.  In the firmware the center A/D value will represent 0 input signal, with higher values representing positive and lower negative.  For example if you have a 10 bit A/D, the 0 input signal value will be around 512.  1023 indicates positive full scale and 0 negative full scale.  To get a signed number in the micro representing the input signal, you have to subtract off this 0 input offset.

A simple resistor divider will give you 1/2 the supply voltage.  Keep in mind that the amp reference input has 100 k&Omega; impedance, so you want to make the 1/2 supply voltage significantly less than that.  Two 10 k&Omega; resistors should do it well enough.  Add around 100 nF to ground to attenuate the inevitable noise on the power supply.  Otherwise, 1/2 of this noise will be added to all your readings.