You may be trying to use a serial number that does not match the version of the plugin. The Download Demo link on mettle.com gives you only the latest version.
You can download an earlier installer from your mettle account, and your license key will work.
If you have an older version of FreeForm Pro (pre 1.75) there is an upgrade option available on the FreeForm Pro product page. You must enter your license key and email address. Your license key can be found in your mettle account.
If you do not remember your password, click on "Forgot My Password" to reset and access your account info.