you first need to encounter it the first time,then you will be where you are now,then when you are both at the same spot on the map,then you must,encounter it in the grass
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if you mean the map marker you can catch mesprit Pokemon of emotions with it hope this helps
After you catch the main legendary (dialga, palkia, giratina) go to lake verity and mesprit will be in the cave in the middle of the lake. Talk to it and then it will flee and appear on your map app.
to catch mesprit in pokemon platinum go to its cave(after beating team galatic and catching/defeating girantina)at lake verity use surf and swim to the cave in the middle. talk to mesprit and it will run away and oak will tell you after it and use your marking map to track it down. best strategy to catch it is get a pokemon with a move like mean look or make it go to sleep. also mesprit is level 50 like uxie and azelf
In map 22 (Ground)
Instead of flying to the nearest city, which makes it run away, walk or bike to its icon on poketch app Marker Map and look around in the nearest patch of tall grass. Mesprit often runs away, so be careful!