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.
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you go to the Dessert Resort and by the "pokemon statues" there's a weird little square and you walk into it and wala you're there!
At one point in the plot, you will be told to go there. Only then will he move and allow you access.
To get the dark stone you need to go to the relic castle to find that it is not there and the you receive a call from professor juniper to go to the museum in nacrene city and you will receive it from the gym leader
The Earthquake TM is in the Relic Castle, but is only accessible after you complete the main story. Yes it is there but you have to encounter volcarona first then go up to again and its in front
if you go down to the very bottom of the relic castle go up the all the stairs and you should be out. hope this helps! =)