You need to answer the questions the three clowns gave you in Jubilife City(or however you spell it) then talk to the guy who first talked to you in Jubilife(or whatsoever)city.
Chat with our AI personalities
from a woman in eterna city's Pokemon city
You get the app from the president of the Poketch company in Jubilife City after winning the Veilstone City Gym battle. In Pearl, this app is used for tracking Mesprit and Cresselia
you touch the screen with your stiylist
there are 25 apps for the poketch.
You use HM6 Rock Smash.Which you get in jubilife city in the cave after you get the poketch