1)
What is the significance that he saw the Mizbe'ach?
1.
Malbim: Divrei ha'Yamim II, 28:23 says that he offered to the gods of Damesek, who were striking him. He chose the form of the Mizbe'ach in Damesek to erect it for the sake of serving Elohei Damesek, to join Hashem's name with idolatry. Also the Kohen Gadol rushed to do the king's word (verse 11).
2)
Why does it say here "Dumesek"? In verse 9 it says Damesek, and in Divrei ha'Yamim II, 16:2 it says Darmesek!
1.
'Rashi' (Divrei ha'Yamim II, 16:2): These are the same. We find like this - Shot, Shevet and Sharbit (Mishlei 26:3, Esther 4:11) are the same.
