In the relationship definition between the tables be sure to click 'Allow creation of records...' on the table the portal is based on.
Thank you for your post.
The reply from "etripoli" is correct (Thank you!). This is how you set the portal to allow adding of records:
1. Pull down the File menu and select "Manage -> Database..."
2. Click on the Relationships tab.
3. Halfway between the "Invoicing Page" and "Line Item", there is an icon. Double-click this icon and an "Edit Relationship" dialog box displays.
4. On one side shows information for the "Invoicing Page", and the other side shows information for "Line Item". On the side that has "Line Item", towards the bottom half of the dialog box, be sure to check the option "Allow creation of records in this table via this relationship".
Now, from the "Invoicing Page" layout, you will be able to add records to the "Line Item" portal.
Thanks to both of you. That was, in the end, incredibly simple. I love this program! It is going to make me look so smart in the office.