Every portal is based on a relationship between the portal's table occurrence (Selected in "Show Records From" in Layout setup...) and the portal's table occurrence (selected in "Show Related Records From" in Portal Setup...).
Open Manage | Database | Relationships and double click the line linking these two occurrences. This opens a dialog where you can select "allow creation of records via this relationship" for the table occurrence selected for the portal. Select that option and you can add new records in the portal as long as the match fields used in the relationship permit it. (If a match field for the portal table is a calculaiton field, this won't work even with "allow creation..." enabled.
Thanks - that resolved my issue!