Ok, it looks like the problem was caused by the fact that I was manually setting a cookie in Firefox to exclude my own traffic, as per Google's recommendations. This caused the Javascript error, and also prevented the cart from showing up. You guys might want to look into this, but for now I've switched to using an IP address exclusion filter instead.
On another note, are we supposed to be able to see Add to Cart or View Cart clicks in Google Analytics? Since the E-Junkie cart code adds onclick handlers to all of the links that I want to track, I can't manually set this up in Google Analytics. I'm assuming the custom E-Junkie integration will allow for tracking which Add to Cart buttons were clicked on by the user.
Is that correct?? What should I see in Google Analytics?