What is Macduff taking about in this quote and how does this relate to the plot?Confusion now hath made his masterpiece. Most sacrilegious murder...

This is a wonderful metaphor and example of Shakespeare
giving a character the language necessary to express the unexpressable.  Just how
affective would it be for MacDuff to enter saying something like "somebody just killed
the king"?   Not very. 

Here MacDuff compares Duncan to a
temple but not just any temple, ""the Lord's anointed temple ".  In other words, Duncan
was choosen by God to be king and that king has been
murdered.

The murder of a king is an unnatural action, thus
the "confusion".  Look at the unnatual things that get reported about the night of the
murder.  Until the murderer has been brought to justice, the world will continue to be
out of order, "unnatural".

This is important to the plot
for that reason.  The rest of the play shows us what happenes under these
circumstances.  People don't trust one another and begin to behave unnatually.  When
MacDuff goes to Malcolm, the young prince does not know if he is a spy sent by Macbeth,
which is why he says the things he said.  He is testing
MacDuff.

Only when Macbeth is killed is order
restored.

Don't forget, the Gunpowder Plot had only
recently been discovered.  There were always plots being hatched to kill the current
ruler, so historically this was also relevent.