Here is the location of a step-by-step explanation on how to use PAPI Web Service (PAPI-WS) to create a work item instance and then run the instance inside an activity in a process running on Oracle BPM Enterprise Standalone.
http://www.4shared.com/file/128249957/9bd6ef61/PAPI-WS_for_Oracle_BPM_Standalone.html
Be sure to use the project that is in the zip file for this example.
To see how to invoke PAPI-WS when using Oracle BPM Studio, go to this thread:
952258
If you need a step-by-step example of how to use PAPI (the Java API) go to this thread:
948979
Hope this helps,
Dan