The first step is to obviously verify that your API and Secret keys are input correctly into the JFBConnect Configuration menu. If they are, the best I can recommend is to try creating a new dummy Facebook application and seeing if that key work. It's possible Facebook has revoked your API key due to some violation of it's terms, but we can't detect that within JFBConnect. All we can do is report the errors it provides to us.
Invalid API key specified.
