Hello Kally,
simply follow these instructions:
- Start Messenger, and sign in.
- Make sure Messenger Plus! is installed.
- Download the Enhancer script from given link.
- Doubleclick the .plsc file to import it.
- Check the security checkbox, and press [Import].
- Open your Messenger contact list.
- Click the scripts icon.
.
- From the menu, choose "Enhancer", and "Preferences".
- Uncheck the checkbox in front of "Enable flashing when a message is received".
- Click the [OK] button.