It would vary on the emulator you're using, but chances are it's "enter/return".
Chat with our AI personalities
hit "start" the select "bag"
open the bag and then grab what you want from it. ARE YOU EVEN A REAL POKEMON PLAYER!
Enter key, then z.
To open up your bag in Pokemon Diamond, simply press X and go to Bag in the Menu. Then, press A.
You press start and select bag, I think?