Windows: http://www.olelo.hawaii.edu/enehana/winkbd.php (caveat that I haven't used this one myself)
Installing the whole keyboard will let you use the kahakō easily as well. I'm sure there are ways to install a couple of shortcuts without the whole keyboard but I find it simple enough to switch keyboards as necessary on OS X.
I also have the Hawaiian keyboard enabled on my phone but I can't remember how I did it. It might have been one of the methods here: http://niuolahiki.ahapunanaleo.org/mod/page/view.php?id=6745
If you are on iOS, just go to the Settings app under General->Keyboard->Keyboards->Add New Keyboard...->and scroll down to Hawaiian. When typing on your device, tap the globe icon on the bottom left corner of the keyboard next to the numbers button to switch languages
I'm sorry I am unfamiliar with Android operating systems.
On the bottom of my Samsung keyboard, just to the right of the "space bar", there is a key with a period mark (.) on it. If you hold that button down, 10 or so common punctuation marks appear, one of which is the single apostrophe mark ('). This is a functionall 'okina for the DuoLingo program.