Add in the local pauseBtn and local pauseBG variables where all the other variables are initialized near the beginning of the code. Preload the btnSound audio after gameOverSound near the top of the script:

-- Place near other game variables
local pauseBtn
local pauseBG

-- Place after gameOverSound
local btnSound = audio.loadSound( "btnSound.wav" )

Within the hud() function and after the scoreText chunk, create another function that will run the event for the pause button. Call the onPauseTouch(event) function. Pause the physics in the game by setting gameIsActive to false and have the pause elements appear on screen:

    local onPauseTouch = function( event )
      if event.phase == "release" and pauseBtn.isActive then
        audio.play( btnSound )

        -- Pause the game

        if gameIsActive then

     ...