unlock iMac with watch

The unlock feature requires any 2013 and later Mac model running the new macOS Sierra using an Apple Watch running watchOS 3 with an iPhone 5 or later.

I, too, cannot get my watch to unlock my late 2013 iMac with Sierra. I get the notice that it is being unlocked by the watch (with the spinning ball), then I'm prompted to enter my password.

It works fine on my MacBook pro.