Welcome to SC since you're new.
1) You really can't without Action Replay since you have to get the Glacidia Flower from this girl in Floraroma Town and she won't give it to you unless you have a special Shaymin given out at the movies when you went to see the 11th movie, and unless they have a similar event in other regions or they alter the terms of gettin' it then it'll be impossible. It's stupid, I know...
2) You'll need Action Replay, but since the game isn't out everywhere quite yet there probably won't be many codes made until it's release in North America at least. I know the code you're talkin' about, but it hasn't been made for Platinum yet. You could maybe go to the Pokemon Action Replay forum and ask cause if you have the Japanese version then somebody there could help.
Siggy proudly brought to you by .Impact!
[insert random quote or phrase that makes the sig space look less bare]