You have a misconception, which signals get's outputted where. If you want to simply read the analog signal of the sensor, you need to use the "control" connector. There you can find the analog signal on pin 2. But this analog signal goes up to 10V, so you need a voltage divider between the sensor and the Arduino, to divide the voltage down to 5V max.
Currently you are using the RS232 connector. RS232 is a communication standard, which transmits digital data, not analog data. Thus you read only digital values (min voltage and max voltage, nothing inbetween). You can use it do talk with the sensor, change configuration settings and reading data digitally. But to actually do this, you need to consider 2 important things:
RS232 is an interface, which involves it's own voltage levels (namely +-12V), which do not fit the TTL level (0 - 5V) that the Arduino Uno uses. You can very well destroy your Arduino by connecting it directly to RS232. For making this work, you need a level converter chip (I think the MAX232 is commonly used for this), to convert the voltage levels to TTL logic. From there on this communication is normally called UART.
You need to understand and then implement the sensors communication protocol (when to send what data), so that the sensor and Arduino are understanding each other
For me it seems, that the analog signal from the "control" connector is all, that you currently need, so for simplicity you can use this. RS232 is used for more control over the functionality of the sensor.