talk to one of the guys in the olivine city cafe. this also works in Pokemon silver and gold.
the olivine city cafe is located to the right of the battle tower.
Chat with our AI personalities
you need to teach your Pokemon rock strength and if the Pokemon you taught is in your party click a on a moveable rock and move it.
Rock tunnel Uncommon
It is in Rock Tunnel.
you cannot get rock climg in crystal because it is disvovered in 4th generation and the game of pokemon crystal compromises the 1st and 2nd generation so you have no chance of getting rock climb also it is of no use here.
Teach them the HM move Rock Climb