You would just need to write $('#listWish').val()
instead of $('#listWish')
.
$("#submitButton").on("click", function(){
$("#sortable").append('<li>' + $('#listWish').val() + '</li>').sortable("refresh");
});
Notice that I'm calling 'refresh' method after adding new item istead of reinializing whole list. For more information read here.
UPDATE:
It could be the case when you call $("#sortable").sortable();
before your markup has been loaded. Try to make it this way:
$(function(){
$("#sortable").sortable();
})
All code inside $(function(){ })
wrapper will be called after all document is loaded.