Here is a working JSFiddle
You can check for the class within your keypress function:
$(function() {
$(window).keypress(function(event)
{
if ((event.keyCode == 13) && ($('div').hasClass('this')))
{
$('#thing').addClass('that');
}
});
});
You may want to change the generic div
to check whether a certain div has the class by its id