Go to your Stripe Account: 

Developers > API keys

Your account API Key is the one called: Secret Key. It is usually hidden and you need to reveal it in order to copy it. 

IMPORTANT: Be sure that your account should be in Live mode. API Key for test data is different and this will no work for production sites. It is very critical that you copy and paste the exact API Key with no additional characters.

Once you got your Live API Key, you should past it in the Monetization > Products, section on the Nunchee Console

Once you finish. You are already done!