Double post...Sorry, but other info I have confirmed, but can't just prove with pics...
You can not get Dialga, Palkia, Giratina, Ho-Oh, or Lugia until you meet ALL of the following requirements......The first three steps have been solidly confirmed elsewhere, but I'm not certain the fourth requirment has been confirmed. The fourth step is up to suspect only at this point.
1) Must capture all three Therion Formes of Tornadus, Thundurus, and Landorus.
2) You must own a copies of Diamond, Pearl, Platinum, HeartGold, and SoulSilver.
3) You only get the legendary mascott Pokemon form each of those carts one time, and they MUST be inserted into your DS/DSLite/3DS/ect during your time in playing the Pokemon Dream Radar game.
4) You MUST have a save file on each of those carts in which you have beaten the Elite 4, AND captured the version mascott legendary Pokemon.
I didn't suspect the fourth step until I tried to get Palkia, and it was nowhere to be found. I reset the save file on it, so no Palkia, or beaten Elite 4 on that save file.
Might want to see if my suspicions can be potentially confirmed by someone else.
*EDIT*
Maybe better pics for a few things??? Five individual pics from the Dream Radar game, followed by some pics as they went into my White 2 save file......
Sygilyph
Swablu
Rotom
Porygon
Togepi
Group picture of those five on the Dream Radar game
Group picture of those five on my White 2 save file
Still not the best quality, but it's better than nothing.