Try this way .
its working on my system i m using office 2010.
Step 1: Open Microsoft Word 2010 Beta and click on the "File" tab. This will take you to the Backstage view. Next click "Help" in the set of tabs on the left.
Step 2: Click “Change Product Key” on the right side of the screen.
Step 3: Enter the Multiple Activation Key (MAK) displayed above. Please check the box to activate automatically or restart the app to activate using the activation dialog box.
Step 4: That’s all you need to do. Entering the MAK key in Microsoft Word 2010 Beta automatically activates all the Office Professional Plus 2010 Beta applications. Click on the "File" tab to return to the Document view.