Do you mean the bar at the bottom of the home screen? If so, you can only have 6 icons on that bar so first thing is to check that you have less than that, then, find the mail icon on one of the screens and touch (not tap) and hold your finger on it until all the icons start to wobble. Then just drag it to the bar and let go, when you're done just tap the home button and the icons will stop wobbling. If you already have 6 icons on the bar you'll need to remove one before adding mail and you do that in the reverse of the above, touch until wobble, drag off bar, drag mail into place.