Why does it say "Zeh Dor Dorshav"?
Rashi: This one (whose deeds are like this), is a generation that seeks Hashem. Radak - it is Yisrael, e.g. in the generation of Shlomo, who build the Beis ha'Mikdash.
Malbim: Even though it seems remote that someone will ascend Har Hashem, for many years have passed 1 and no one has ascended or descended to join the upper and lower beings, now a complete generation is found that reached this level. They all ascended; all are Dorshei Hashem, Mevakshei Fanecha and at the level of Yakov 2 , who saw a ladder on which they ascend to Elokim and angels descend below, join the upper and lower beings. Then they were individuals, the Merkavah of the Shechinah 3 . Now it is an entire generation!
I.e. since Moshe's generation, when the Shechinah descended to the Mishkan, until David's generation. They were worthy to build the Beis ha'Mikdash, just David could not build it, so they needed to wait for Shlomo. (PF)
Surely they were not truly at the level of Yakov. David erred to think that he was at the Avos' level (refer to 51:6:3:5)! Yerushalmi (Pe'ah 1:1) says that David's generation fell in war, because there were slanderers among them! Perhaps after they saw what happened to David's enemies, they repented. (PF)
The Avos are the Merkavah; the fourth leg was Adam ha'Rishon, until David (who was like a Gilgul of him) came in place of him (based on Yalkut Chadash David, 63 - PF).
Why is it written Dorsho, and we pronounce it Dorshav?
Radak: The Kesiv (singular) refers to the Klal (the generation). The pronunciation (plural) refers to the Prat (individuals).
Why does it say "Mevakshei Fanecha Yakov"?
Radak: The author addresses Hashem. They seek Your face in the Beis ha'Mikdash. Yakov is Yisrael; all his seed are called Yakov - "v'Atah Al Tira Avdi Yakov" (Yirmeyah 30:10). Meforshim say that it should say Zera Yakov, just 'Zera' is omitted; we need not say so.
Why does it say "Selah"?
Radak: Refer to 3:3:4:1-3.
Malbim (3:1, 21:1) says that "Selah" divides between different parts of a Mizmor.
