After passing through Brun Way, you find the gatehouse leading to Dendemille Town, where you meet up with Professor Sycamore.
Meet the Professor
When you go east from the Dendemille Town entrance, Professor Sycamore and Dexio meet up with you. The Professor tells you about the legendary pokémon of Kalos, which goes to sleep in a hidden place after its lifespan. Dexio will do some research into its whereabouts.
After Dexio and the Professor leave, Trevor runs up and says he's going to the Frost Cavern to find more pokémon. Frost Cavern is to the north.
Get an O-Power
Mr. Bonding is the Dendemille Pokémon Center. He gives you Accuracy Power.
Use your Dowsing Machine near the entrance of the Pokémon Center to find a hidden Heal Ball.
The girl in the black dress near the Pokémon Center sells Moomoo Milk.
To the southeast, you find Route 17: Mamoswine Road. The only way to use the road is to ride the Mamoswine, but the Mamoswine has gone to the entrance of Frost Cavern and refuses to come back.
Back in Dendemille Town, go east from where Trevor is standing near the Route 15 gatehouse. Go east past the stairs and into the house. The Move Deleter and Move Reminder are here. The Move Deleter can make a pokémon forget a move. The Move Reminder knows all of the moves that a pokémon learns when it levels up, and and if you give her a Heart Scale she can make a pokémon learn any of these level-up moves, up to its current level.
Go south from the Move Deleter/Move Reminder's house, across the bridge, then use the Dowsing Machine to find a hidden Nugget in the bottom-left field. Pick up the Big Root that is sitting nearby.
Now go back to the Move Deleter/Move Reminder's house and go up the stairs to the left of it. Go northeast, ignoring the set of stairs on the left. You will reach a house in the northeast corner. The old man will evaluate your Mountain Kalos Pokédex. Be sure to come back to him when you have seen 70 pokémon in the Mountain Kalos Pokédex.
Go west from that house, under a walkway. Use the Dowsing Machine to find a hidden X Speed in the northwest corner.
Go south from here. Go into the southwest house and talk to the kid to get TM42 Facade. The girl in this house will give you a berry if you have the TM that she asks about.
Since you can't go any farther until the Mamoswine goes back to Mamoswine Road, you should use the north exit of town to reach Frost Cavern.