The workflow participant operates on a single work item basis. Therefore he needs to designate which piece of work he wishes to do.
Step 1 Fetch the workitem list for the login user.
View the call details for fetching the workitem list.
Step2 Call WMGetWorkItem call for getting the workitem. For this call you need a valid Process instance id and workitemid from his worklist.
Through this call you can designate an activity to the workitem. The viewer may be selecting a work item from the worklist.
Assigning Process Instance Attributes