you have to migrate it from your emerald game if you have one. so you can't catch it from Pokemon diamond.
or ruby and Saphire
I woodent i would just by a action replay and get the code for one
if you are talking about rayquayza in Pokemon you need to get it from your game boy emerald game
Rayquayza
Go to the Sky Pillar
no you have to migrate from emerald, sapphire, or ruby
On top of sky pillar.
there is no way to get one as starter Pokemon unless you use a laptop, computer and use the cheats
You can only trade from emerald. You have to migrate it from one of the Sinnoh games and then catch it at Pal park.
Pokemon diamond
talk to spinda after defeating rayquayza 2 to 3 missons later
Pokemon diamond Pokemon diamond Pokemon diamond
you beat groudon then xatu will contact you and say that a shooting star is falling. then you get ready get your sleep, then goto the hill of ancients and alakazam will give you a gem that will transport you to the clouds and you have to go about 30 floors and battle rayquayza
you cant be a Pokemon in Pokemon diamond