I think when you get all of the 8 badges in the johto region and beating the Pokemon league for the first time.
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NO The Pokewalker was made especially and only for SoulSilver and HeartGold, so the only way you could walk a Pokemon from your Platinum game is if you traded that Pokemon to SoulSilver\HeartGold and then transfer it onto the Pokewalker
You could find wild Growlithes in Route 48.
Diamond, Pearl, Platunum: You can only go to victory road. Heart Gold, Soul Silver: You could pick Victory Road, Kanto or Mount Silver(Kanto and Mt. Silver roads will be closed(Kanto Closed for time being and Mt. Silver needs Scary Strong Pokemon)
I don't think you can get Latios in Heartgold legitimately. I'm pretty sure you can only get Latios in Heartgold by using an action replay code, such as the Pokemon Modifier Code. You could also trade it over from Soulsilver, I think.
You can get a Munchlax by breeding it with Snorlax holding a Full Incense, or you could transfer it from the Poke Walker.