Agile WebDev: Iteration D1: Ajax malfunction

I am trying to get through Task D: Iteration D1 of Agile Web Dev.
I have gone through all of the D1 section, but when I click the 'Add to
Cart' button, the sidebar cart does not update. But, if I do an <F5>
refresh, the added products show up in the cart.

The development log shows the following:

Processing StoreController#add_to_cart (for at 2006-12-18
16:39:57) [POST]
  Session ID: af9f55eacb478a8c1c7d161c06a52a3d
  Parameters: {"commit"=>"Add to Cart", "action"=>"add_to_cart",
"id"=>"2", "controller"=>"store"}
  e[4;36;1mProduct Columns (0.000000)e[0m e[0;1mSHOW FIELDS FROM
  e[4;35;1mProduct Load (0.016000)e[0m e[0mSELECT * FROM products
WHERE ( = 2) e[0m
Rendering within layouts/store
Rendering store/add_to_cart
Rendered store/_cart_item (0.00000)
Rendered store/_cart (0.00000)
Completed in 0.01600 (62 reqs/sec) | Rendering: 0.00000 (0%) | DB:
0.01600 (100%) | 200 OK [http://localhost/store/add_to_cart/2]

I am such a confused newb that I am not even sure if this is telling me
of an error.

Please help me figure out where my error is and how to fix it.