Not exactly the secret it used to be so have at it!
Should take about 20 minutes drive or so from Waikiki/Town area.
Go West on the H1 Freeway towards Pearl Harbor.
Find the Safeway on Moanalua Road at the Waimalu Plaza.
There's a ditch between the Safeway and Waimalu Elementary School.
Get in there and go towards the Freeway!
Google Map this shit, you'll find it.
It might be dirty/wet/muddy depending on recent weather but when rideable its pretty fun and definitely worth the trip.