go to the cave in the middle of the lake near twinleaf town. talk to it and professor rowan will show up. mesprit escapes and will roam around sinnoh. you should have a thing in your watch thingy to track it down. follow it and go to a place where you can catch Pokemon in the area. catch it. try not to fly there because that will raise the chance of running away
you cant catch it in platnium but you can trade it
anywhere
transfer it to your pal park
you continue on to the next gym
At spear pillar but to encounter him you need Azure flute
No, you cannot catch Mesprit in either of those games. To get Mesprit you can:Trade over the GTSTrade from Diamond, Pearl, or Platnium
you cant catch it in platnium but you can trade it
Catch better Pokemon
no ditto is there
you cant
Yes you can.
you get it from a fossil
no
anywhere
Catch a wild magby
jiroxy ball...
You can't you would have to catch one in Pokemon Heartgold or Pokemon Soulsilver and trade it to Platnium