Why does he call to Hashem?
Radak: He calls amidst his affliction.
Malbim: It says in Shmuel II, Perek 15 that when David fled Avshalom, the Kohanim took Aron ha'Bris with them. The king commanded Tzadok to return the Aron to the city. He trusted that Hashem will answer him far [from the Aron] like close. David says, I will call to Hashem wherever I am, and He will answer me me'Har Kadsho, from far like from close.
Why does it say "va'Ya'aneni" (past tense)?
Radak #1: The past is used in place of future. It is as if it says v'Ya'aneni; there are many cases like this.
Radak #2: It truly means the past; David was confident [that Hashem answered him]. Or, Ru'ach ha'Kodesh came on his tongue; this is correct. All the Mizmorim were said with Ru'ach ha'Kodesh. In Nevu'ah, the past is often used in place of future. It is as if the matter was already done, since it was said with Ru'ach ha'Kodesh.
What is "Har Kadsho"?
Radak #1: It is Har ha'Moriyah. Even though it was not yet Kadosh (before the Beis ha'Mikdash was built there), they had a tradition that the Beis ha'Mikdash will be there. This is why when David fled Avshalom, he ascended Ma'ale ha'Zeisim (Shmuel II, 15:30), and there he bowed to Hashem (ibid., 32), for from there one can see the place [of the Mikdash].
Radak #2: It is Har Tziyon, where the Aron was.
Radak #3: It is Shamayim, like "Mi Yishkon b'Har Kadshecha" (15:1).
