I tried everything described in the thread without much luck. I could not get the checkbox in the settings to show up on the latest model of MacBook retina 13" and my watch was untrusted on the appleid. I got it working after unpairing and pairing my watch. After that the checkbox immediately showed up in the settings.
Just go to My Watch app, and tap on your watch at the top. Then click info button on the right and select Unpair Apple Watch. Then follow instructions on the screen. Unpairing and pairing, and restoring from backup might take a while, so be patient. It took probably 40 mins for me.
After the sync had been complete, the check box magically appreared in the settings and I was able to unlock my laptop without problems. Also my watch finally became a trusted device on the appleid site.
Tried it several times, seems to be working so far. I hope it helps.