you get it from a person in Solacean town.
In jubilife city.
on the touch screen after you receive the app.
you come with it!
It makes the Pokemon you tap 'cry'.
get the Poke tech App Poke happyness
The Pokemon Diamond and Pearl series can be watched on the official Pokemon TV website or by watching them using the Pokemon TV app for iOS and Android. You can also download the Diamond and Pearl episodes legally through the Pokemon TV app, but only up to 3 episodes at a time.
its included in the default applications
You get it from a women on 1FL of the Pokemon Centre in Eterna City.
use the dousing app. around the lobby
There is no such app, there is only an app which can show you where roaming Pokemon, such as Mesprit and Cresellia are, called the Marking Map (which can be obtained from the president of the Poketch company after obtaining the Veilstone badge).
In Pokemon Diamond the Friendship Checker (also called the Happiness Checker) app is #6.
Eterna City's Pokemon center. Some random dude gives it to you.