Ok. I found out why my plug-in didn't show up. After the plug-in is installed, two settings need to be changed manually:

Enable One-Page Checkout?
False => True

Enable Guest Checkout?
False => True

Now it works, other than the warning log generated:

--> PHP Warning: var_export does not handle circular references in /includes/classes/OnePageCheckout.php on line 744.

When debug is ON, I received the following warning:

--> PHP Warning: var_export does not handle circular references in /includes/classes/observers/class.checkout_one_observer.php on line 690.

Line 567 warning is gone, by changing the code provided by Lat9 in the previous posts.