He is at pastoria town.He is next to the safari zone.You have to talk to him then he will run away.You have to chase him,but when you're chasing him your rival(the kid with yellow hair) will show up and battle you.Beat him and chase the galactic guy.Eventually he will lead you to the lake.
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He is on the route right of lake valor
go to pastoria and beat the gym leader there.(you have to beat veilstone first)chase the galactic guy to lake valor and beat him. Cynthia will give you medicine to cure the psyduck and go to celestic town.
in valor lake front after u chase the guy in pastoria city keep walking up a little till u see a blond girl she will give a secret potion and then you go to the Pokemon then give them the secret potion and then they will move hope it help
you have to find barry in the lake by snowpoint city save him, then go to veilstone in front of team galatic HQ. talk to the guy standing there and he"ll give you the key
If you mean the guy who won't let you go to sunnyshore city, then you have to defeat Cyrus at the spear pillar. If you are using platinum, after you get out of the disorientation world. Check again after you do that!