You can achieve this by adding/appending code below in update() method of javascript which is attached to gameobject inside level ( level which contains ad)
if(Input.GetKeyDown("escape") && !isPaused)
{
isPaused = true;
bannerView.Hide();
}
else if(Input.GetKeyDown("escape") && isPaused)
{
bannerView.Show();
isPaused = false;
}
//And in your pause button, you can have something like this ( if it is a GUI button)
if(GUI.Button(Rect(0,heightofbutton, Screen.width/20, Screen.height/20), pauseimage, myButtonStyle1))
{
isPaused = true;
}
//Just an example. You can modify it as per your needs.