Flexiforce pressure sensor with positive voltage
The piezoresistive effect just means the resistance of the sensor changes with pressure.  With their circuit, the -5 V supply will be inverted and amplified with the gain varying with the sensor resistance.  The output voltage will vary from 0 to 5 V.

Instead of using a -5 V supply, you could just connect the bottom of the sensor to ground, and connect the positive input of the op-amp to a 2.5 V bias voltage.  Then the voltage at the output would vary from 2.5 to 5 V.

Depending on what you're using it for, you could just connect it as a voltage divider, and then follow that by an inverting amplifier with an adjustable gain and DC bias so that you can get 0 to 5 V for whatever pressure range you want.