To integrate your online ordering with Stripe, you need to get your Secret API Key.
Login to your Stripe account, Find the Developers tab on the left hand side & hit API keys
You'll want to reveal your secret key not your publishable key, copy & paste it on the Settings>Order Payment screen after selecting the Stripe payment gateway.
IMPORTANT: Make sure that you are not using test data. If this is the case, switch the View test data toggle. Test API keys will not work in your Orders2me account.