In Pokemon Diamond for DS...?

Once you get all your gym badges and you beat the team galactic leader he will say he is going to Mt. Coronet to do something. So go to the top of Mt. Coronet and beat the team galactic leader and the other commanders again and catch Dialga. After that the guy blocking the way to Sunnyshore city should be gone. Hope i could help!

if you have already gone to team galactic headqusters go to the moutain enter through hearthome then keep going up there would be the leaders and cyrus face them and then you get dailga and palkia then sunnyshore should be open