Hi S s,
Please check the PPDS Global settings in customizing to see when to send the orders to ECC. You shiuld sselect value 'Always create transfer events' or some thing similar. This is the normal default setting. This in conjunction with 'Publication' and an Integration Model' for Planned orders should work.
you can confirm by creating a planned order and save it in APO. On refresh the planned order should have a number from ECC.
Thanks,
Subash