There are two methods of retrieving your license.
If you became an XMD Life-Time Member before 2/12/2017, you will have to request your license through the form here. http://michaeldunnam.com/xmd-toolbox-license/. This will require your email that is associated with your Gumroad account.
If you are a new member, you will have received an email with your purchase with your License Key attached. You can alternatively log into your Gumroad account and in your library under XMD Life-Time, it will have your License listed.