To open up your bag in Pokemon Diamond, simply press X and go to Bag in the Menu. Then, press A.
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open the bag and then grab what you want from it. ARE YOU EVEN A REAL POKEMON PLAYER!
hit "start" the select "bag"
It is not possible to get the Seal Bag in Pokemon Diamond. The Seal Bag currently has no use and is likely something left over from development. It possibly used to have the same role as the Seal Case but got changed during development.
You press start and select bag, I think?
You can't store items on Pokemon Diamond and Pearl, as your bag now can carry an unlimited supply of items.