Read data from strain-gauge Labjack, Arduino
Since your sensor has a piezoelectric output, it has a charge output that will provide a voltage that swings both positive and negative with respect to ground into a high value load resistance.  I would try tying one output to both the "IN A-" input of the amplifier and the VREF pin, with the other output tied to the "IN A+" pin, with switch #6 on and switch #5 off.  This will allow the amplifier to remain in the active region as the voltage goes negative.  The belt spec sheet does not indicate whether a load resistor is built into the belt sensor, so you may need a high-value resistor between "IN A-" and "IN A+", in the 100K to 1M range.  The resistance you choose will affect the low frequency response, so you may have to experiment to find the right value.