how do i unlock my mac book pro with my i watch

The watch needs to be updated to Watch OS 3, and the MacBook to macOS Sierra. Then go on the MacBook to settings --> security & privacy --> and select "allow apple watch to unlock your mac."

Automatically unlock your Mac with your Apple Watch - Apple Support

Note your need a mid 2013 or later Mac with Sierra

In addition to the replies below, note that you have to have an iCloud account which has 2-factor authentication