The value proto service now allows you to submit either your PCB123 design or gerbers. If you are going to submit your PCB123 file you will need to zip your pcb123 file and select the Upload File button. After uploading the file select the PCB123 file for each of the layers in the layer mapping section to proceed with your order.
If you choose to use the upload your files individually option you should map your PCB123 file for each layer you expect on your PCB.
Please contact customer support if you have any questions.