To catch Dialga you have to have the Admant Orb. To obtain this Orb you must have the HM Waterfall. You obtain this after beating the last gym. (To get to this gym you must have atleast one of the last Pokemon, Azelf, Uxie, or Mespirit, just a heads up) So after you have the HM go back to Mt. Corunant. Climb up until you find a waterfall. Swim up and go into a cave. There you will find the Admant Orb and another orb (used to catch Palkia). Then go up to the top of the Mountain to Spear Pillar. Before walking out give the Admant orb to the first Pokemon in your party. Walk out onto Spear Pillar and you will see a shiny flying slightly diamond shaped blue shiny thing. Walk over to it and press the "A" button. You will then face Dialga. Have lots of Ultra/Great balls. It will appear at Level 70.
you cant catch it in platnium but you can trade it
no ditto is there
you cant
no
Yes you can.
you get it from a fossil
Catch a wild magby
You can't you would have to catch one in Pokemon Heartgold or Pokemon Soulsilver and trade it to Platnium
No, you'll have to trade it from Platnium. Which in Platnium you find it in the Rock Peak Ruins.
top of mt cornet
transfer it to your pal park
you continue on to the next gym