I have two questions please help. 1.How do you get to Sunnyshore when..

1. You can't yet

2. Go to Mt. Coronet (through hearthome and catch dialga)

You must first head to Mount Corenet and climb your way to Spear Pillar. Here you will face Team Galactic again. One you crush them for good, you will have a shot to catch Dialga. After that, the Guy blocking the route to Sunyshore City will be out of your way.

To answer both questions 1 and 2, you need to go to the top of Mt. Coronet if you were paying attention to what about 3 Galactic members said, and then what Cyrus said a few times.
I answered this question too many times.

Make sure you have a bunch of Max Repel, at least 40 Dusk Balls, some Max Potions, Full Restores, and Revives, and Pokemon that know: HM03 Surf, HM04 Strength, HM06 Rock Smash, and HM08 Rock Climb.

Now, fly to Oreburgh City. Go north and up the mud slide, and then east to go in the Mt. Coronet entrance. Go across the water using surf, and then up the rocks using Rock Climb. In the next room, just attempt to go as north as you can, and then exit the first part of Mt. Coronet. You end up along the sideline of Mt. Coronet. Along the mountainside, there are two caves to go in. One is a dead end, and the other is the right cave that you need to go in. So, if you go in a cave that leads to nothing, exit and look around for the second cave. Once you find the right one and follow the path, battling a few grunts along the way, you'll end up along the mountainside again. This is a different portion, though. This part is a little tricky and gets to people when they end up in front of a Grunt that says he has no pokemon. Uh, dur, that's the wrong cave! Exit the cave and find the right one, which from all I remember is all the way west of this mountainside. Once you find it, just follow the one-way path until you exit the cave. Once you're out, make sure you have at least 30 Dusk Balls, some revives, full restores, and max potions, and heal your team up. SAVE YOUR GAME!! Do the following when it's night.
*WHEN IT IS NIGHT IN-GAME*
Battle the two grunts, heal your team up, and battle the two commanders with your rival. Your rival will heal your team up himself, and now battle Cyrus. When you beat him, and after the mini-clip heal your team up and do not do anything else but SAVE YOUR GAME. Once again, make sure it is night, or wait until is night, and battle the level 47 Dialga. When his health is in the red zone, and, if possible, asleep or paralyzed, start throwing Dusk balls to catch him. You should eventually catch him, and I just answered both numbers 1 and 2.