You are looking in the wrong place. The right hand side, where you are expecting the 'ProcessInstanceAttribute' to show up, is the Process Context pane, which shows the list of your data objects in the process design.
'ProcessInstanceAttribute' node will always show up in the output mapping of any activity, which you will need to map with a created data object of string type in your process context:
The output of Start event will never show you the 'ProcessInstanceAttribute' node. It will only show you the parameters of the associated WSDL. Rest of the activities, you will see it in the output mapping.
So, if you want to fetch the process instance id after start executes, may be place a mapping activity after the start event and fetch the instance id in the output mapping.