go through the oreburgh gate. take a Pokemon that has a rock
smash, surf, rock climb, and strength with you. go north from the
entrance, then surf east, continue walking east, climb the rock
wall and go up the stairs.
You have to enter the mountain at Celestic City east-ward then
just climb upwards. To get to Lake Acuity you have to search around
the mountain for HM08: Rock Climb and use it on a tall mountain and
there you go! But you have to defeat the Snowpoint City gym leader
to use Rock Climb.