Define n-step approval for purchase order

Hello,

I am having some troubles defining several levels for approval of a purchase order ( BUS2201 ).

I have already implemented BAdI BBP_WFL_APPROV_BADI, but it doesn't seem to execute the BAdI when the PO is created. I'm feeling some obvious step is missing, but I don't know which one...

Any help would be very much appreciated.

Thanks in advance,

António