the page you sent only goes to line 273, it does not include lines 359 and 358 referanced in your message.
From your description, when you added the cart display, you added a read only display, not an editable display.
When you add the eCart display manager, make sure to select the editable display option
as for the enter key functionality, the cart display form will submit the first button when you hit the enter key.
f you want the enter key to have a specific function, like updaing the cart, you will need to re arrange the code for the buttons:
<input type="submit" name="eCart1_Continue_100" id="eCart1_Continue_100" value="Continue Shopping" />
<input type="submit" name="eCart1_Clear_100" id="eCart1_Clear_100" value="Clear Cart" />
<input type="submit" name="eCart1_Update_100" id="eCart1_Update_100" value="Update" />
<input type="submit" name="eCart1_Checkout" id="eCart1_Checkout" value="Checkout" /></td>
so that the update button is the first button:
<input type="submit" name="eCart1_Update_100" id="eCart1_Update_100" value="Update" />
<input type="submit" name="eCart1_Continue_100" id="eCart1_Continue_100" value="Continue Shopping" />
<input type="submit" name="eCart1_Clear_100" id="eCart1_Clear_100" value="Clear Cart" />
<input type="submit" name="eCart1_Checkout" id="eCart1_Checkout" value="Checkout" /></td>