There is no way your dumb. We all have our own strengths and weaknesses. Also it's wrong of your step dad to get frustrated just because you didn't get what he was explaining.
But please know that you are not dumb. I myself was a slow learner, I didn't pay attention in school, I had no desire to. But I call myself a late bloomer, and I actually think I have in some respects surpassed my peers in some sense. You'll get there, perhaps you are also a late bloomer =)