I didn't find my License key because I purchased from Elegant Themes - Divi Marketplace

The version at Elegant Themes - Divi Marketplace does not required a license key to use. The differences between our own version and the version on Divi Marketplace is just the license key. If you see the notice in your dashboard asking for License key then you can simply Dismiss it because you'll get updates automatically.

Hope that solves the mystery!