you have to capture Dialga to do that you must go to spear pillar and battle boss Cyrus then captre it finally you can go to sunyshore city
Volkner
Sunyshore city, where the last gym badge will be.
catch dialga the go to sunnyshore
In Sunnyshore city same as pearl/diamond.
the fifth gym on Pokemon diamond is the one in herthome city,
The first gym in Pokemon diamond is in Oreburgh City
After you do the second to last gym it becomes available
in the grass in the last city the 8th gym leader
Volkner, he will be at the lighthouse, 2nd floor. Talk to him and he will go to the gym waiting. Uses electric types.
Volkner and he has raichu Electivire Jolteon and i think a Lantern in Pokemon platinum not sure about diamond.
you go to sunnyshore city but don't go to the gym go to the lighthouse and you will find the gym leader talk to him and then he will go back to the gym so you can fight him
The seventh gym in Pokemon Diamond, Pearl, and Platinum is located in Snowpoint City, which is to the East of Lake Acuity.