There is a link to the company's integration guide on the page you linked which is very explanatory.
These sensors show a decrease in resistance with increasing applied force. I have no idea what you mean by "connected to the breadboard (with resistor)", but the most simple way to read this device would be to connect it to a multimeter set to read resistance. Once you know the resistance you can calculate the approximate force from the curve on page 5 of the integration guide (shown below).
So, if you are reading 500 Ohms of resistance across the force sensor, then the approximate mass on the sensor (force $\neq$ mass, argh) is 4 kg. In order to get a more accurate measurement though, you will need to calibrate the sensor with a range of known masses since every sensor produced by the company will be slightly different.