On the Shield the magazine release is on the left side of the frame for right handed operation. The release push button is not interchangeable as is some of the other models of S&W M&P pistols.
I just use my middle finger to hit the mag release. It's pretty easy with a bit of practice. I can't get the proper angle to use my thumb and a right side release anyway (without completely changing my grip on the pistol).
there is a video on youtube, i believe where a guy discusses adjusting his grip so this doesn't happen. i did the same, and it has never been an issue for me (i'm a lefty).