You have to trade it with heartgold or soulsilver
after beating the game, get to pal park and migrate from emerald
afte completing the shinoh pokedex, go to Prof. Birch and their should be a cut scene where Prof. Oak comes and gives you the nantional pokedex
After you get the National Dex, go into Victory Road Cave. Go into the door that you couldn't go into when you weren't Champion. Use Defog, then Surf on the first body of water you see. You should find one. It works better when you battle the non-Lapras Pokemon. Also if you keep going you will arrive at 224 were you can find Duskull and Dusclops. And you need catch it on a Friday.
I'm sure there's a code that allows you to do that.Or, you and your friend,relative could put your secret bases next to each other and go in and out of the bases with the flags until you get Platinum.
All I did was slather every tree in the game with honey and came back to each 10 hour later. Honey trees are the only way you can catch a Burmy.