1.Add jQuery library (It's missing in your code)

2.Add `keydown` event-listener on `text-box` and check enter key is presses or not? if yes, the call the `addListItem()` function

Just add below code inside `$(function(){..});`

 $('#new-text').keydown(function(e){
 if(e.keyCode === 13){
 addListItem();
 } 
 });


Working snippet:- 


<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

 function addListItem(){
 var text = $("#new-text").val();
 $("#todolist").append('<li><input type="checkbox" class="done" /> '+text+' <button class="delete">Delete</button></li>');
 $("#new-text").val(' ');
 
 }
 function deleteItem(){
 if($(this).parent().css('textDecoration') == 'line-through' ) {
 $(this).parent().remove();
 }else{
 $(this).parent().remove();
 }
 }

 function finishItem(){
 if($(this).parent().css('textDecoration') == 'line-through' ) {
 $(this).parent().css('textDecoration', 'none');
 }else{
 $(this).parent().css('textDecoration', 'line-through');
 }
 }
 
 $(function() {
 $('input[type=text]').keydown(function(e){
 if(e.keyCode === 13){
 addListItem();
 } 
 });
 $("#add").on('click', addListItem);
 $(document).on('click', '.delete', deleteItem);
 $(document).on('click', '.done', finishItem);
 });

<!-- language: lang-html -->

 <script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
 <!DOCTYPE HTML>
 <html>
 <head>
 <title>My Page</title>
 <meta http-equiv="Content-Type" content="text/html;charset=utf-8" />
 <link rel="stylesheet" type="text/css" href="style.css" />
 <script type="text/javascript" src="jquery.js"></script>
 <script type="text/javascript" src="TodoListJquery.js"></script> 
 </head>
 <body>
 <h1>To-Do List</h1>
 <ul id="todolist">
 <li><input type="checkbox" class="done"/> Clean house <button class = "delete">Delete</button></li>
 <li><input type="checkbox" class="done"/>Buy milk <button class = "delete">Delete</button></li>
 </ul>
 <input type="text" id="new-text" /><button id="add">Add</button>
 </body>
 </html>

<!-- end snippet -->