I just started working with your plugin and keep seeing a message box saying that the license key is not valid. It states to go to the account page, but there is nothing mentioned about a license key anywhere. I searched the forum and don't see anything here either.
If you have a paid account you can find the license key in the account page (see the top menu of this site). If you have not a paid account, probably there is something set in the license key field inside the main settings of Newsletter: just empty the field.