You can no longer get it in Pokemon platinum unless you trade it with someone. It was obtainable through a wifi event with an item known as the Member pass (Which is no longer available through Wifi)
try finding a really good ball with as many as possible and get darkrias hp as low as possible and just keep throwing the balls
Route 224 Poke Radar
Go the spring path and walk around until you find deoxys and use your poke radar if you can
To catch a pokemon, you require a Poke ball, which you can buy from a poke mart.
right after you get national dex
I suggest you use poke radar and migrate poke. and all to finish that national pokedex and if for some reason if u didnt catch Lake legends or palkia and Dialga talk to lucas and he'll say Some weird signal coming from spear pillar. (lake legends found in lake cave)You can attempt to catch again! and catch giratina In the snow city and catch regice if u have other regis. Put them in ur team and go in the ice cave.Talk to lucas also for advice on poke radar.
poliwag?
You can only obtain Darkrai through Poke Transfer in Pokemon White/Black.
use poke balls on it
Darkrai can be caught in any kind of Poke Ball, but Ultra Ball has 4x the catch rate by itself, which is pretty high.
Lower its HP until it's at the Red marker. Find it again, then put it to sleep with a fast Pokemon. (Suggest darkrai) Get a variety of poke balls and throw them at the Pokemon to try and catch it. If you have used poke balls a lot of turns then throw a TIMER BALL.
no its only avalible on a pokemon event
Route 224 Poke Radar
there will be an event where you can catch arceus coming soon.
Can't be done mew is not a catchable poke in this version
Master ball
fuego ironwoks (use poke radar)
route 222 with poke radar