After Dragonspiral Tower, you should go to Relic Castle. Get to Route 4 by either going south from Nimbasa City or north from Castelia City. Then look for the road leading west, near the freeway that goes over Route 4. This road takes you to Desert Resort.
If you don't remember how to get to the Relic Castle entrance, or you haven't checked it out yet, here is how to get there. Stay on the solid sand near the eastern wall of the desert. Eventually you'll reach some slopes. Head west along those slopes. Once you see the statues (which are Zen Mode Darmanitans), walk toward them to reach the stairs down into Relic Castle.
Read more about Desert Resort if you haven't been there before.
As you approach the entrance to Relic Castle, Cheren shows up. After speaking to him, go inside.
To find Alder, you need to fall down into certain sand traps until you reach the bottom of Relic Castle. The first step is to head west from the entrance, being careful to WALK, not run, across these sand pits. If you run, you'll fall in, and these sand traps lead to a dead end. If you haven't fought the Psychics here yet, check the Desert Resort section for tips on fighting them if you need help.
Otherwise, just head west and go down the stairs. These stairs used to be blocked because the worker said that the sand hadn't been moved out of the way.
Ryoku, one of the Seven Sages, is here. Past him, you must fight a Team Plasma grunt who has an L34 Watchog and two L34 Krokoroks. For the Watchog, use Litwick, and for the Krokoroks, try using Swanna.
After defeating this grunt, fall into the sand trap to get to the next floor. Just walk into the middle of the trap to fall down into it.
Get the Max Potion nearby, then fight a grunt who has an L35 Scraggy and Watchog. Use Ducklett against Scraggy, attacking with Flying moves, and use Litwick on Watchog.
Walk slowly across the sand pit to get to the next grunt, who has an L36 Garbodor. Use Golett against it, and attack with Ground moves.
Walk slowly across this next sand pit to get TM30 Shadow Ball. Then you can fall into the pit.
The nearby grunt has an L36 Krokorok. Use Swanna against it. Go slowly across the next sand pit to get a Max Revive.
To get a Sun Stone, drop down any of the sand pits except for the bottom-left one. There is a Super Potion hidden near the Sun Stone. It's a dead end and you'll have to go back up to B2F to continue, so you might want to get it later.
Otherwise, fight the grunt who is guarding bottom-left sand pit. She has an L34 Liepard and two L34 Trubbish. Against the Liepard, try using Ferroseed, Scraggy, or Klink against it. Against Trubbish, use Golett.
Get the PP Up from beyond this sand pit, then drop down into the sand pit.
The first grunt here has an L35 Watchog and Scraggy. The next one has an L35 Trubbish and Liepard. Drop down the sand pit to proceed.
Alder is on the lowest floor of Relic Castle. When you get to B5F you will see Alder and Ghetsis. Cheren will drop in after you. Ghetsis will have a long discussion with all of you about Team Plasma's desire to release all trainers' pokémon. Alder has no choice but to go to the Pokémon League and fight N. After Ghetsis pushes past you, you and Alder and Cheren leave the castle. Then Professor Juniper will give you a call and ask you to come to Nacrene City.
Go to the Nacrene Gym. Alder, the two Professor Junipers, and Bianca are there. After some dialog, Lenora will step out. When Alder asks if you're prepared to be the one to fight N if Alder is defeated, say yes. Then, you will receive the Light Stone if you're playing Black, or the Dark Stone if you're playing White! Your next destination is Opelucid City, so fly back to Icirrus City and prepare to head north to Route 8.