You will need Several HMs: Strength, Surf, Defog, and I believe you will also need rock smash. Go through the Celastic Town entrance, so you don't need to use strength twice. Go up, and use strength on the boulder above. Go down the hall and turn left. Now if you see two grunts, you are not far enough in the storyline. Go down the stairway. Use Defog, and if you want the pokeball, use rock smash. Go up and use surf. Now follow the stairwell, and you should be outside. The first trainer you encounter will have a Aimbom and a Porygon.
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First, you will need Surf, Rock Smash, and Rock Climb (and Waterfall if you want to climb the waterfall). Enter Mt. Coronet from the Hearthome City or Oreburgh City side. Make your way northward through the cave. You will have to go outside in the snow at one point. Keep rock climbing till you find a cave that leads back into the mountain, and the next room should have the waterfall.
follow the game.you have to get through mt.coronet.get a ice type,cause it'll hail to your way there. by:poke-expert rookie pearl
You've gotta migrate it through Pal Park from Fire Red & Leaf Green, Which is to detailed and impossible at this point in time to go through. (Nintendo event)
Go to Route 217 outside of Snow point City. (You have to go through MT Coronet to do so) And there is a mean behind a house and he will give you the HM: Rock Climb. He also gives you an icicle shard I think.