Open Manage | Database | Relationships
Find the relationship linking your layout's table occurrence to the portal's table occurrence.
Double click the relationship line linking to the portal's table occurrence.
Select "Allow creation of records via this relationship" for the Portal's table occurrence.
Now you can add new records in the portal simply by adding new data in the blank row just below the last row in the portal to display data.
Note 1: this requires that the portal's Match field in this relationship not be a calculation field.
Note 2: With some layout designs it works better to use a button with a script that creates a new, blank related record in the portal for you. For one thing, with the appropriate sort order, this new blank record can appear at the top of the portal row and thus eliminate the need to scroll your portal records each time you need to add a new record once you have more portal records than rows in your portal.