Pre-Set Up:

Confirm that the "list ports" button outputs your serial ports: here. If it doesn't go through the instructions again.

Confirm that the serial port is running on your arduino (open serial monitor in the Arduino software)

Some sample code for a sensor on pin 0 in the Arduino

void setup(){
 Serial.begin(9600);
}

void loop(){
  int input = analogRead(0);
  Serial.print(input, BYTE);
  delay(50);
}

Try this on one of the examples: here
Do your own

create a folder with the JSSerial.jar and jsserial-applet.jnlp along with your own HTML/JavaScript file.

edit the jsserial-applet.jnlp to put in your own "href" and "codebase" values depending on where you put your files.

See this for a barebones sample. If it runs correctly when you press the button, the serial monitor values will run on your screen.
Please note: if you have more than one javascript serial page open it will confuse the sytem, ONLY ONE PAGE OPEN AT A TIME.