You wanted to set up email notifications to be sent when purchase orders are approved.
SAP Business ByDesign
You have to set up work flow rules for those scenarios that does not exist under Home ->Subscribe to tasks
Go to Application user management work center
Go to work flow rules view
Follow steps given in SCN community blog in below link
Refee the help center document
KBA , AP-POP-PO , Purchase Order , AP-SIP , Supplier Invoice Processing , Problem