No worries. First you have to open Image>>Adjust>Brightness/contrast. Then go to File>>New>>Script. A new window will open which allows you to run scripts. Now in this window, go to Language>>Select Java script. Finally run the following commands.
The window to adjust the intensity should appear on the top left of your screen after you run this. If you are looking for the threshold tab, it will be under Image>>Adjust>>Threshold. If your threshold tab is not visible just change the term B&C in the above script to Threshold and then run it.