Initially, I had no problems getting my Apple Watch to unlock my mid-2013 rMBP. Then it just stopped working; like others, I would see it attempting to unlock, then it would just give up after about 10 seconds and prompt me for a password. I tried everything I could think of to get it to work. The last thing I tried was cycling Handoff on my Mac, iPhone and Apple Watch and then rebooting the iPhone and then Apple Watch. After that, it's worked quite a few times. Don't ask me why, as all the other settings were property established and unchanged from when it initially worked. I thought maybe the iOS 10.0.2 messed it up, but now it's working.
Here are the official instructions from Apple's help docs in macOS Sierra:
Unlock your Mac with Apple Watch
When you’re wearing your Apple Watch, you can use your watch to unlock your Mac after waking it, instead of entering your password.
Note: To use Auto Unlock, your Mac (a mid-2013 or later model) and Apple Watch must have macOS Sierra and watchOS 3 installed, respectively. You must be
signed in to iCloud using the same
Apple ID on both devices, and
two-factor authentication must be turned on for your Apple ID. Make sure your Apple Watch is unlocked.
- Turn on Auto Unlock on your Mac: Choose Apple menu > System Preferences, click Security & Privacy, then select the “Allow your Apple Watch to unlock your Mac” checkbox.
Open Security & Privacy preferences for me
If necessary, click the lock icon
at the bottom of the pane, then type an administrator name and password.
- Unlock your Mac: Wake your Mac from its idle state by pressing any keyboard key or, on a portable, by opening the display. The screen indicates your Mac is being unlocked.
System Information can confirm if your Mac supports Auto Unlock. Choose Apple menu > About This Mac, then click System Report. In the sidebar, click Network, click Wi-Fi, then look in the Interfaces section for Auto Unlock.
If you’re the administrator for another user on your Mac, you can log in to their user account and turn on Auto Unlock for them, as long as their Apple ID uses two-factor authentication and their Apple Watch has watchOS 3 installed.