To add a column to the left of the cell, click Insert Left in the Rows and Columns group. To add a column to the right of the cell, click Insert Right in the Rows and Columns group.

Insert is used to add a row.
To insert a row, right-click on a cell or row that is not the header row. Pick any cell in the table and right-click to enter a column.
Point to Insert and select Table Rows Above or Table Columns to the Left to insert a new row or column.