Security header is not valid
means that the API Credentials you entered are not correct.
The most common cause of that error is using Sand box API Credentials when sandbox mode is not enabled, or vice versa.
double check the API Credential that you entered.
NOTE: The API Username and password is not the same as your paypal account login. see the "Signing up with the PayPal payment gateway" tutorial on the learning resources page if you need more details: