Where does Lady Macbeth wait for Macbeth after the murder of Duncan in Macbeth?

Lady Macbeth waits for Macbeth outside Duncan's chambers after he kills Duncan because she wants to make sure that Macbeth does it right.

Macbeth was not really committed to killing King Duncan.  Even though he was excited about the possibility of being king when he heard the witches’ prophecies, and was angry when he learned that he was not Duncan’s heir, he lost his nerve when he returned home. 

Lady Macbeth seems to predict this.  When she gets his letter, she knows that he will never follow through.

Glamis thou art, and Cawdor; and shalt be
What thou art promised: yet do I fear thy nature;
It is too full o' the milk of human kindness
To catch the nearest way: thou wouldst be great;
Art not without ambition, but without
The illness should attend it … (Act 1, Scene 5)

Basically, Lady Macbeth is saying that Macbeth does not have it in him to kill.  He is too nice, and while he has ambition, it is not enough to stay his moral reservations about killing.  Basically, he is not ambitious enough.

It turns out she is right.  Macbeth falters and makes arguments with himself about why he should not kill Duncan.

He's here in double trust;
First, as I am his kinsman and his subject,
Strong both against the deed; then, as his host,
Who should against his murderer shut the door,
Not bear the knife myself. (Act 1, Scene 7)

Macbeth has talked himself out of the murder. He feels bad, because Duncan is his guest, and his relative.  He cannot bring himself to kill him when he has done nothing wrong.  He is a good king, and a good friend.

For this reason, Lady Macbeth takes charge.  She plans the murder carefully, and admonishes Macbeth for not being manly enough when he objects.  He follows her orders for the most part, but she gets upset with him when he does not leave the knife behind.  It is part of her plan to frame Duncan’s guards.  This is why Lady Macbeth has to watch her husband carefully and wait for him. She is sure that he will either chicken out or mess up.  She is close enough to hear what happens.

He is about it.

The doors are open, and the surfeited grooms

Do mock their charge with snores. (Act 2, Scene 2)

Although Macbeth is ambitious, he is also weak-willed.  Until Lady Macbeth pushes him and forces him to kill Duncan, he waffles.  If not for her intervention, he never would have acted.  Once he kills Duncan, however, he has set off on a path he cannot come back from.  He is so desperate to maintain his throne that begins a murderous spree against perceived enemies.  Eventually his paranoia results in him losing his fighting spirit and letting Macduff kill him.