I want to capture ectoplasm in Platinum.

For ectoplasm, take a plant Pokémon with the move "Sweet Scent" and go to the old castle (in the Vestigion forest... in one of the rooms you'll see a frame with two red eyes inside). Walk around the room using Sweet Scent. Be patient, it won't come out of its hiding place easily.
Prepare a weak Pokémon to inflict small damage, and then you just need to throw Hyper Balls or Dark Balls.

For the legendary birds, you need to have finished the league and obtained the national Pokédex. Go talk to Professor Oak who is located in Vestigion in the first house at the bottom left (the house to the left of the Pokémon gym). He will tell you that the three Pokémon have been seen in Sinnoh. In the applications, take the map. You will see them moving. It’s up to you to run into them.
Small hitch: they flee on their first turn. So take a fast Pokémon with the move Barrage to prevent them from escaping and Fake Out to not knock them out. If your Pokémon has less than 185 in speed stats, it’s over. You can always have it hold a Quick Claw, but the chances of your Pokémon hitting first are very slim...
Personally, I use a level 65 Scyther. Fast (205 speed) and with good attack stats (187 attack). Of course, I boosted it with Carbos when it was young =) .
Prepare a good stock of Hyper Balls or even Timer Balls. Good luck!