Here are my terminologies, so you understand what I was walking about:

Opener: the remote control I place in my car, which has 3 rectangle buttons.
Two Receivers: Mounted bottom of two sides of garage door frame
Controler: component on the garage door motor, which has a button (someone called learn button), a green light which lights up when you press the learn button, and 3 terminals with all sorts wires attaches to the.
Wall Unit: mounted on the wall in garage which use to open/close garage door from Garage.

1) I can use wall unit to open/close garage door, so the motor works.
1.5) I made sure the remote was not "locked" from wall unit.
2) Press button on opener, the green light on opener lights up, so the battery in the opener works, it is a brand new battery I put in yesterday.
3) Close door from garage, then immediately standing below the garage door, the door open up again, with garage door light blinking, Could I say the receivers also works?

4) This may not related, but it is worth to metion, there are 2 garage door lights on both side of control unit on the top of garage ceiling. A month ago, one of the light doesn't work, I thought it was a bad light bub, so I changed a new light bub, but it still didn't work. Since the door still works, so I didn't pay much attention. This broken light is on the same side as control unit while all 3 terminals and the learn button are.

5) I did open the opener, there is no switch on the opener.
6) I also unplug the power, and replug in after 5 minutes. It still doesn't work.

My question,

1) If it turns out tht I had a bad opener, can I just go buy another opener (from Sears/Craftsman), can it be easily programed?

2) Do you where I can download Sears/Craftsman opener user manual?