Apparently, this sensor is analogue. That means, you can just call SensorPort.S1.readRawValue() and you get a value between 0 and 1023.
The curve relating the raw value to the force applied (in newton) is given on that page:http://www.hitechnic.com/cgi-bin/commerce.cgi?preadd=action&key=NFS1074
(It's amusing, how they measure force in grams, not newton)
I just looked at the RobotC driver suite. I didn't find any code for doing the interpolation. And maybe, the exact curve is different for every inidividual sensor anyways.