I'm glad Fizz's comment was of help and that's what was missing! I'm going to answer to get it off of the unanswered list.
First you need strength, which is available from Iron Islands.
Iron Island (Japanese: 鋼鉄島 Steel Island) is an island off the western coast of Sinnoh. It is only accessible via ship from Canalave City.
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At the entrance to the mine, you'll meet a man named Riley, who will give you HM04 (Strength). He mentions he is training inside and proposes you join him. While receiving the HM is the only required part of your mission here, you'll find some good items, get valuable training, and an extremely rare Pokémon if you explore the whole mine.
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Later on, you'll get Rock Climb when you visit Route 217.
Route 217 will be difficult to navigate due to both the slow movement and heavy blizzard besides having to deal with Hail conditions in battle. Also there are ninja themed trainers hiding in the snow so look for mounds in the snow to find and face them. Use your Dowsing Machine to find hidden items while you are at it. When you arrive you'll encounter the Gym Leader Maylene who is traveling to Snowpoint City not properly dressed as a form of exercise. Out of all the items you can find as you are exploring the route make sure to go to the first house in the middle of the route to find HM08 (Rock Climb). You will still need the next Badge to use it outside of battle.
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If you get lost again, I highly recommend reading the Platinum Walkthrough on Bulbapedia. It will tell you where to go, step by step :)
Hope this helps!
