First, your computer should be connected to the internet so that your PC or Mac computer can detect and activate your Office purchase. 1. Open Word. 2. Select Get Started or Sign In. If you have a product key instead, click I have a product key link. 3. On the Activate Office dialog box, type your MS account email and password to sign in or select Enter a product key instead. 4. Follow the on-screen prompts to finish the activation process.