Cynthia should gIve it to you near lake valor. lake valor is in the middle of pastoria and veilstone city.
The Secret Potion is a Key Item which can not be discarded, sold or used until the required event with the troubled Psyducks.
There is no legitimate secret potion cheat for Pokémon Diamond, Pearl and Platinum version. One way to do this would be to use an Action Replay.
Ok, after you get the Secret Potion from Cynthia, proceed on Route 210. That should take you to the Cafe Cabin. Walk to the upper left corner, where the Psyduck are. Press "A" when it asks you if you want to use your secret potion. Note: even after you use the Secret Potion it will still remain in your Key Items section of the bag Hope it helped!!
Pokemon league champion Cynthia will give it to you when you run into her at lake valor(near pastoria and sunyshore)
rt. 210 is above Solaceon Town. it is block by psyducks until you can cure their headaches with the secret potion
easy
cynthias grandmother gives it you after bthe bomb scare which is after the psyduck ness which means u need to get the secret potion to cure them which you get from Cynthia after battling the galactic grunt your meant to be stalking in pastoria city.(pick up the secret potion at the valor lakefront
you get it l8ter in the game after u beat the water badge i think its the 6th badge
go to the left of moo moo milk shop, then give the secret potion to the psyduck :)
from Cynthia
You get the secret potion by talking to the pharmacist in Cianwood City. His house is in the south, south east of the gym.
Use a potion.