After Team Plasma's attack on Opelucid City and other areas, Cheren says to look for them near Humilau City. To get there, go north from Undella Town.
Reach Humilau City
To get to Humilau City, first go to Undella Town. You could fly there or just walk. There is ice in many places around Unova that you might need to take a detour around.
In Undella Town, go west from the pokémon center. Hugh will call you and tell you to go through the Marine Tube to reach Humilau City. The Marine Tube is the blue building with stairs leading down. Go down the stairs to enter the Marine Tube.
There are no trainers or items in the marine tube. Just go north and enjoy the scenery on the way to the exit. When you use the north exit, you'll go through a gate building. Talk to the Parasol Lady there to get TM42 Facade. Then exit the gate building.
When you reach Humilau City, you can see Hugh to the north. Talk to him and he will tell you to defeat the Humilau Gym leader before you try getting the DNA Splicers back.
Go to the northern part of town and try to go into the first house. Marlon will hop out of the water. He's the Gym Leader. He'll go back to the Humilau Gym, so you can battle him whenever you're ready.