Visual Composer license can be overviewed and managed by visiting WPBakery Support Portal - support.wpbakery.com. Once you activate Visual Composer you will be automatically redirected to Support Portal in order to choose/approve license you want to activate.
Note: If you have just purchased Visual Composer it can take up to 30 minutes to synchronize Envato and WPBakery Support Portal to retrieve your license.
Important: Visual Composer activation does not require you to know your license key. Upon activation process you will need to authorize yourself via Envato Account and Visual Composer will retrieve your license details automatically.
Starting from October 2015 there's no need to find your license manually to enter it manually in Visual Composer. All that is now automated: Video Demo.