You can only get Regirock by transferring a Regirock from Pokemon Ruby, Saphire, or Emerald to the Pal Park I believe.
In Diamond you can't catch Regirock, you have to migrate it from Pokemon Ruby, Sapphire, or Emerald.
You cannot
its only in pokemon platinum
you can but i don't know how yet
yes,with action replay
You can find Regigigas in Snowpoint Temple in Pokemon Diamond, you will need to bring Regice, Registeel and Regirock for Regigigas to waken.
you don't need regigigas for registeel. you need reegisteel, regirock and regice for regigigas on Pokemon pearl or diamond
To get a Regirock, Regice or Registeel you have to migrate one from Ruby, Sapphire, Emerald, Leaf Green or Fire Red.
You can get Dialga Mesprit Uxie Azelf Giratina and if you somehow get regirock, regiice, and registeel on to diamond you can also get regigigas
you need regice, regirock, and registeel in your team when you fight him
You can only get a regice, regirock, and a registeel by migrating them from ruby or Sapphire. If you migrtate these Pokemon you can also catch regigigas.
migrate regirock, regice, registeel, and then you can access the regigas (migrate from emerald, ruby, or sapphire)