The force sensor is a sensor for "through variable", so you'll need to connect in series with the element on which you'd like to measure the force. Looks like you want to measure the force applied on the mass, so you need to place the sensor in series with the mass.
Also, it looks like you are expecting some gravity applied on the mass. This won't happen automatically. The mass block in 1D models implies no gravity. You'll need to specify gravity using a force source. It can be placed where you have your force sensor right now.