The marking map app can be found in Jubilife city from the Pokétch company. Talk to the owner and he will eventually give it to you.
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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).
Go and talk to the owner of poketch in poketch company's building he will install the marking map app in your poketch
if your talking about the marking map wich shows you were the Pokemon that fled is, that comes with the poke watch it's the one with all the shapes like a triangle hope this helped! -chibi
When a Pokemon is travelling around (mesprit, cresselia) the marking map will show a little face. Wherever the face is on the map, you will find the Pokemon. The Pokemon wil not be standing there, it will be hiding in the tall grass or even on water..
use the diamond etc from down the bottom and just6 drag it to where you want it.