Sunnyshore City.
Defeat Volkner first...
To get HM7 (Waterfall) in Platinum you can get it at Sunyshore City talking to Jasmine at the north of the city.
HM7 contains the move Waterfall.
it depends where and far stuck u are so you find it in the blizzard in snowpoint city next to a house at the top.
Waterfall
Hm7 is waterfall, sorry im not sure where i found it though. :-)
hm7 waterfall is in the cave of orgin, sootopolis city, you have to go to the magma lair otherwise they won't let you in
It's in the Cave of Origin in Sootopolis city.
hm1 hm7 hm2 hm3 and hm5
Beat the 8 Gym Leader, then go to Sunnyshore city.you should see a girl with two pony tails. She will say she is from Jhoto and start talking to you.at the end she will give you HM7(Waterfall) and says it's the key to the Pokemon league.
In Pokémon Ruby Sapphire and Emerald versions, HM7 contains the move waterfall. You can find the HM in the Cave of Origin.
I believe it is either HM6 or HM7.
It's in the ice path that leads to Blackthorn City.