Figured it out! :-) Added a calculation field in the context of the parent. Whew! Slowly getting this...
Did you try just draggin it out of the portal and putting it on the layout? It works on my sample db.
For field options you should have "Total of" selected, the field in the portal as the Available field.
We want to create a function that will sum values of a particular table based on the current ID of a record we are on? We have added a portal to show the child records but are having a heckuva time figuring out a way to display a summary of the dollar amounts in the portal.
Create a summary field in the child table. Display the summary field on the layout of the parent table. The summary field will display the correct sum.