If you entered a product key during the installation, Minitab 16 will attempt to activate the software automatically at launch.
 

If you did not enter a product key during the installation:

1. Select Activate Minitab.
2. Click the Next button.
3. Enter your product key.
4. Click the Next button.
5. Select Automatic Activation.
6. Click the Activate button. A confirmation message is displayed that your software was successfully activated.
7. If you receive another message, contact Minitab for support at http://www.minitab.com/support/licensing/activation.aspx