Seank6,
I assume you have been flying 'toy' flight sims until now, which simplify all the things. X-Plane is a real player on the market and is (almost) as real as it gets. Hence, you need to know the altimeter value and just click on the right side of the altimeter setting knob to increase the set value or the left side to decrease it. Try it the hard way, gives you much more joy than just clicking 'b'. Same as flying the real thing...