The laws regarding medical marijuana differ from state to state, check the link below for information on going legal in Hawaii:
Becoming A Patient in Hawaii

It seems in Hawaii you do have to register with the government, however, that is only optional in California.

That is, of course, Hawaii state law, Federal law does not recognize any state medical marijuana laws but the DEA has stated they are not targeting individual users. Instead, they are mostly focusing on the co-ops and growers that provide medical marijuana. I'm not even sure if there are any co-ops in Hawaii, in California many cities and counties are banning them from even opening up, and some police officers are even helping the DEA shut down co-ops. But as an individual patient, you shouldn't have to worry about too much.

It is confusing so don't feel dumb for thinking so, hopefully the next president will end this crap and just let states to individually decide themselves if they want medical marijuana, (which is how it should be), but only time will tell.

P.S. Jesus Christ, I was just reading the laws for simple possession in Hawaii, 30 days incarceration and $1000 fine?? WTF? I would have thought Hawaii would be laid back with their pot laws...well, be safe then!