Your template's CSS file is compressed, so I can't give you the exact line or file to modify. The problem is the below style though:
body .btn [class^="icon-"], body .btn [class*=" icon-"] {
display: none;
}
That's saying "Don't display button icons". You'll need to remove that display:none style, and once you do, the icons will show just as expected.
Let me know how that goes, and good luck!
Alex