I looked at form 1237 and I can't see any sign of anything you have done to add the icons to the buttons.
Can you tell me what you have done and which tutorial you were following.
As you cannot modify the markup CRED generates for the fields and buttons, I would expect you to need to add some custom JS that modifies the form markup once it has been generated, but I'm not seeing any of that—nor anything else.
Thanks for the clarification, it helped me track down where I was expecting to see the icons used and appear.
On a page on the front-end where I would expect to see them (e.g. hidden link) they are not, and they are not because your theme is not loading Bootstrap and so the glyphicons are unavailable.
See the screenshot from the browser console which shows the stylesheets loaded on the page—Bootstrap is notably absent.