Synshrone city is not a real city in Pokemon Diamond.
Go South of Hearthome City
walk there
Go to Celaceon City northeast from hearthome
you go to sunnyshore city you go to sunnyshore city you go to sunnyshore city
you have to get dialga after you could go to sunnyshore city.
go to eterna city
Velistone City
You have to go east of eterna city.
it is not realy possible to get there
Cerulean City
In Pokemon Diamond\Pearl, you go to the Veilstone City Gym. In Pokemon Platinum, you go to the Hearthome City Gym.
its to the write of hearthome city after tons of trainers:(