The only way to add more icons is to edit the index.php file of the theme. I believe the Donate button for this theme was created using a widget in the Custom 1 position; if you removed that widget, that would explain the disappearance of the button.
NOTE: Please remember to always post a link to your site when having issues with something on the front-end; I can't even begin to diagnose the cause of a problem if I can't see what's happening and inspect the code with
Firebug.