He is in the Pokemon Mansion on the route below Hearthome City.
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You don't, Mr. Backlot doesn't show you Manaphy but there's a book in his room that will show you a picture of Manaphy as well as add it as a "seen" Pokémon to your Sinnoh Dex.
151 is Manaphy and can be found by looking at the book in Mr. Backlot's room in the Pokemon mansion on route 212.
you get castform in the trophy garden just talk to mr backlot its down to chance tho and the list grows when you get the nationel pokedex
Each day, if you talk to Mr. Backlot in the mansion south of Hearthome, he'll say there is a certain Pokemon there. For the next two days, that Pokemon will appear randomly in the trophy garden. Igglybuff is one such Pokemon.
Once you have the national pokedex go to the Pokemon mansion and save in front of mr backlot then talk to him until he tells you there was a ditto in the trophy garden and use the pokerader (you HAVE to have the national pokedex)