What is the meaning of "Lo la'Adam Darko"?
Rashi: The Yetzer ha'Ra that You created in him, a person does not have strength to make his way succeed. If not via Your mouth, the enemy cannot destroy Your Bayis.
Radak: Man is not in control of himself; You change his desire. You caused Nebuchadnetzar to change the place he came to attack (refers to 10:22:1:2 and the note there). There is a Kamatz under the Lamed in la'Adam, for it refers to the known person - Nebuchadnetzar.
Malbim: The choice of path is not to man, rather, only to Hashem. And even if he picks a path, going on it is from Hashem. If so, You can stop Nebuchadnetzar from choosing to go to Yerushalayim - rather, he will go to Rabas Bnei Amnon. And even if he already chose this path, You can stop him so he cannot go!
Why does it say "Lo l'Ish Holech"?
Radak: A person does not go in the world on his intended path; he does not know what will come to him. The preparation is not in his hands, like David said "me'Hashem Mitz'adei Gaver Konanu v'Darko Yechpatz" (Tehilim 37:23).
Malbim: Refer to 10:23:3:3.
