All of the reviewers so far seem to have gone down the Upolu Airport road to get there. Alternatively, a mile or so west along the highway you can take Old Coast Guard Road instead, which is paved all the way to a parking area at the ocean. The parking area is rough and rocky but that’s the only difficulty. From there it’s less than a mile of walking along a scenic coastal trail past an old closed Coast Guard LORAN station to King Kamehaha’s Birthplace, and a bit farther to Mo’okini Heiau.
Both are impressive structures with a wild and remote feeling, which you may have to yourself. There is no on-site interpretive information at all, so do your reading beforehand and come with a respect for the Hawaiian history, culture and religion they represent.