To activate your copy of Office 2016 for Mac you’ll need to visit Office.com/setup and follow the next steps:
Step 1: Obtain a Product Key from your order from MSOffice Works, copy the product key.
Step 2: Using your Mac, go to the browser and type in the following : https://office.com/setup
Step 3: Read the text page that appears on the screen. At the bottom of the page, there is a space for a 25-digit number. This is where the product key number should be inserted.
Step 4: Type in or paste the 25-digit number. Proofread the number to be sure it is correct.
Step 5: Click on the box that says Get Started. Create a Microsoft Account or use your existing account and begin installing Office 2016 for Mac.
You need to create or use your existing Microsoft Account to login this step is mandatory.
Here is the steps with screenshots:
Step-1
http://office.com/setup
Step-2
Step-3
Thanks
MSOffice Works Team.