Step 1: Click on Start Menu Control Panel Programs and Features (or Add/Remove Programs, depending on your operating system). Click on your product, e.g. Microsoft Office Professional Plus 2010, then click “Change” on the top menu.
Step 2: Select the option to “Enter a Product Key.”
Step 3: Enter the appropriate product key from the top of this page and click “Continue.” On the next dialog, click “Install Now” to install your key. You will see a progress bar page and then a dialog box saying configuration has been successful. Click “Close.”
Step 4: Close and restart the application and follow the on-screen instructions to complete activation (an internet connection is required). After restarting your Office application once more, click File tab | Help to see your activated status.