In general, when you receive application configuration errors, we recommend that you use our built-in Autotune Configuration Tool. To launch this go to your backend > Components > JFBConnect > Overview > click on the Autotune button. The Facebook App tab provides recommendations on what the domains, URLs, etc should be set to, so see if there are any values that are marked as red and try to use our recommendations.
The error you're seeing is usually that one of the values for Domain, Site URL or Canvas/Page Tab URLs are incorrect.
Let us know if you're still having issues after running Autotune.
By the way, I tried to create a user on Firefox for Facebook and it went through but was waiting on admin approval. This was from my macbook though, and not iPad/iPhone. I figured I'd try again on one of those after Autotune is run.
-Melissa