You have to fly to Veilstone City and go into the headquarters. Before you go in the looker from jublife City come and tells you to meet him in the Galactic Warehouse. Go there and he'll use a key and the door will open and now you have to go down the stairs and eventually you'll find Cyrus talking to a bunch of grunts. Hope this helped cause i have every single Pokemon in the national dex in Pokemon platinum.
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acuity lakefront i think
Lake Acuity is to the east of Snowpoint City, and you pass Acuity Lakefront on your way to Snowpoint City from route 217.
Route 217, Acuity Lakefront, Lake Acuity (Night only). Listed as 'uncommon'.
On Route 217 and by Acuity Lakefront (by Snowpoint City).
Use a Pokemon to Fly to Acuity Lakefront from Celestic Town. If you don't want to Fly whatsoever, then go west into the tallest mountain, and then going north all the way, then past the whole snow and blizzard.