Who is commanded to seek Hashem?
Radak: Bnei Galus. Malbim - this is because the salvation depends on Teshuvah and good deeds.
When is Hashem found?
Refer to 55:6:3:1.
Rashi: He is found before the decree was made - then He says "seek me!"
Radak citing his father: When you seek Him with all your heart.
Radak (from Sifri Nasa 42): Before the decree is sealed, He is found [to nullify it].
Targum Yonasan: Seek fear of Hashem while you are alive. Radak - i.e. repent before death, for then He is found and close, but not after death, for then there is no Teshuvah - "Ki Ein Ma'aseh v'Cheshbon v'Da'as v'Chachmah bi'She'ol Asher Atah Holech Shamah" (Koheles 9:1).
Malbim: Before you sin, He is by you.
Our verse implies that we can find Hashem only sometimes. It says also "who is like Hashem Elokeinu whenever we call to Him"!
Yevamos 105a: He is always found to a congregation, but not always to an individual. He is found to an individual during the 10 days between Rosh Hashanah and Yom Kipur.
Why does it add "Kera'uhu bi'Hyoso Karov"?
Malbim: If you sinned, call to Him to return to you before He leaves you due to your sins.
