Can I assume you are using this 3rd party jQuery plugin?
I've never used this plugin before but I can see you're not doing things just like their coding examples so I wonder if that's your problem. Their example binds to the form's submit event and calls ajaxSubmit() within that. So mirror the examples on the plugin's website exactly to get it working.
You should also try using alert() to find out what the value of "data" is in the callback function. But first find out if the ajaxSubmit is even working (you can also stick alert() calls everywhere to find that out too!).