Your activation key is sent to the email address you used during purchase immediately after the transaction is complete. Search your inbox (and spam folder) for "NESmaker" or "The New 8-Bit Heroes." Official Support:
The is a unique license key required to unlock the full features of the NESmaker game development tool. Unlike a standard product key, it is applied within the software after installation to verify the license and ensure the software is genuine. Obtaining a Code Nesmaker Activation Code
CODE INVALID — 0 OF 3 ATTEMPTS REMAINING Your activation key is sent to the email
But you hit the paywall.