I have loaded the project and tried to export the centroid xy file, but I am getting an error "Missing Either Top or Bottom Components layer.

It appears that you have loaded the Gerbers successfully, unfortunately the Gerber files themselves do not contain any information about the components -- Gerber is just geometric data without intelligence (i.e. lines, circles, arcs,). 

To generate the Centroid data you must use the command Build Components:

1.  Here's a quick tutorial showing how to generate the components:
http://forums.numericalinnovations.com/post/gerber-to-centroid-7733448

2.  Here's a great help guide explaining the centroid file generation in detail:
http://numerical-help-guide.s3.amazonaws.com/understanding-the-centroid-file-r2-2.pdf 

3. There are several other more detailed tutorial movies in the FAB 3000 user forum
http://forums.numericalinnovations.com/?forum=106290
http://forums.numericalinnovations.com/post/generate-xy-centroid-file-tutorial-6737754


If you require further assistance, feel free to send me the sample gerber files, and I'll be happy to demonstrate how to get it started.

Thanks again.

Best regards,
Simon