Yes, you need a op-amp circuit to output a usable DC signal for a Arduino analog input pin. Below is their recommended 'conditioning/calibrating' circuit.
It's a pain to have to come up with that negative 1 volt for the sensor and calibrating the feedback resistor looks to be a pain. It's an interesting sensor, but they sure didn't make it very user friendly for interfacing.
http://www.tekscan.com/flexiforce-sample-circuit
Lefty