The full quote from McNeill's book page 100 is:
Disturbers are to be rebuked, the low-spirited to be encouraged, the infirm to be supported, objectors confuted, the treacherous guarded against, the unskilled taught, the lazy aroused, the contentious restrained, the haughty repressed, litigants pacified, the poor relieved, the oppressed liberated, the good approved, the evil borne with, and all are to be loved.
Searching around for the word "pastor" in the pdf's of the 11-volume Augustine Sermon collection of the New City Press edition, I accidentally found what could potentially be the source, in Sermon 77C (Volume 3 page 338, pdf here). But this may not be the source, since the beginning of the quote seems to be taken from 1 Thess 5:14b:
warn those who are idle and disruptive, encourage the disheartened, help the weak, be patient with everyone.
Unfortunately what survived seems to be only a very limited FRAGMENT (see Notes, page 339) (emphasis mine):
Correction with love
When a person sees someone who is living a bad life, making a gift perhaps to the Church, and doesn't correct him, he is running away in spirit. What constitutes "running away in spirit"? Being afraid. Fear is a kind of inner flight. Why is he afraid? Because he's a hireling.2 He's afraid the other person may take being corrected badly, and not give what he usually does. He sees the wolf coming, that is to say, the devil breaking the neck of the person who is living a bad life, and he flees in spirit, he refrains from salutary correction, being full of fear. But the one who is a true shepherd and cares for the sheep doesn't let him
get off scot-free, and does what the apostle says: Correct the trouble-makers, encourage the faint-hearted, etc. (1 Thess 5:14).
So a pastor, or anyone who calls himself a pastor, shouldn't imagine it's a godly act of not returning evil for evil, when it's much rather a matter of returning evil for good. After all, the other person, ruffianly sinner though he is, is giving the Church some of his goods; but the bishop, the so-called pastor, is returning him evil for good by withholding from him suitable correction.
But all this, of course, has to be done out of love; so because people some times regard those who correct them as their enemies, that's why, after saying Correct the trouble-makers, he added, Encourage the faint-hearted. Perhaps, you see, after being corrected, the person may be very upset and begin to lose heart; that's when encouragement is needed. Give support to the weak, in case they fall through weakness. If weakness makes them stagger, charity should clasp them to its bosom in support. And after saying this he added in conclusion, Be sure not to return anyone evil for evil (1 Thess 5:14-15). So correction, if it is offered, is not evil. But what does the good sheep say, when it is corrected by its superior? The just man will reprove me in kindness (Ps 141:5).
The search continues ...