@Kimela, I have done a fair bit of Milky Way Photography. Your best bet will be as close to 30 August (the New Moon) as possible. If you are more than a few days before or after this, then moonlight will tend to wash out the Milky Way. Shortly after sunset at that time of the year in Hawaii, the Milky Way will be in the southern sky running almost vertically from South to North.
If your camera can shoot in manual mode, try this as a starting point (It will NOT be the final setting, but it should get you into the ball park.) Use the widest lens you have, and open it up to its widest aperture (the lowest "f-stop" number). Set your ISO to 3200 and your shutter speed to 25 seconds. Auto focus will not work, so you will have to focus manually. (Point at a bright star or planet and make the dot as small as possible.) Use a tripod and take your picture. It will take a little trial & error, but it isn't really too hard especially if the skies are clear.
As an alternative to Mauna Kea, if you can't get up there, a second option that would make a very cool back drop would be if you can find a spot where you can see the lava flow in the background and get the Milky Way in the sky with the orange lava in the background.
Here is a shot I took in Arches National Park a few years ago:
One thing to keep in mind whether you go up to Mauna Kea or to Volcanoes National Park is that you will be climbing significantly from Sea Level. Make sure you allow sufficient time to off-gas before going to either location or even better, do this before you go diving.