You can add or delete rows and columns to the table in the visual using the buttons in SmartPanel.
Begin by clicking a cell in the column to the left of where you'd like to add a new column.

Then click Add Column in SmartPanel to add a column to the right of the selection.
Begin by clicking a cell in the row above where you'd like to add a new row.

Then click Add Row in SmartPanel to add a row beneath the selection.
Begin by clicking the cell(s) of the row(s) or column(s) you'd like to delete.

Then click either Delete Column or Delete Row in SmartPanel.