(I'm not sure if it's the same for diamond as it is for pearl) Occasionally visit the pokewatch maker in oreburg (?) city and he will have, hopefully, made new apps. And be sure to talk to everyone on your way, especially in houses, because they will also provide new apps. I don't know if there's an AR code for this....don't think so.
Chat with our AI personalities
the most you can get is in jubilife, in the pokewatch building after you defeated the elite 4.
If you have the happiness App. for your pokewatch thing... you can see how much your Pokemon loves you (how happy they are).
Go to the pokemart ( Velistone and the Pokemon League should have a supply of dusk balls, otherwise you may come across them by having a Pokemon with the ability "PICKUP" or finding items using the dowsing app on the Pokewatch.
Its name says it all. It simply spins and stops at your command.
You can find articuno in the poketch marking app when he's all around