Oak's Letter

Press L+R+Select while walking into a pokemart to activate code then talk to the guy in green.

Platinum:
94000130 fcff0000
62101d40 00000000
b2101d40 00000000
0000b5d4 00000009
0000b5d8 00000000
d2000000 00000000

Diamond and Pearl:
94000130 FCFF0000
B21C4D28 00000000
b0000004 00000000
0000AA3C EDB88320
2000AA68 00000009
d2000000 00000000

I do not take credit for these codes.