I think your talking about HeartGold/SoulSilver. In Pokemon HeartGold/SoulSilver you're not capable of going to the Sinnoh region, only Jhoto and Kanto are avaliable.
You can not. It is impossible for Heartgold and Soulsilver.
Not in Heartgold/Soulsilver.
There is no sinnoh region in heartgold or soulsilver, it is only Johto and Kanto.
There is no Sinnoh to unlock in either game. If you want to access Sinnoh, you must buy Pokemon Diamond, Pearl, or Platinum Versions.
you can't go to sinnoh in Pokemon soul silver.
You dont get Arceus in HeartGold. You need to trade one from another HeartGold or SoulSilver or trade one from the Sinnoh region games. WARNIG: IF YOU HAVE AN ARCEUS ON BLACK OR WHITE YOU CAN NOT TRADE IT BACK TO THE JOHTO OR SINNOH GAMES.
Sinnoh is a region only available in Pokemon Diamond, Pokemon Pearl, and Pokemon Platinum.
unofrtunately, you can't unlock the hoenn or sinnoh regions in heartgold or soulsilver. but i hear that the Pokemon executives are going to come out with a game that encompasses all the regions soon. (I think it starts with the hoenn region, then goes to kanto, then johto, then sinnoh.