There are 2 ways to insert a new element to the decision table context menu. A new element can be added by clicking on the "plus" signs or by accessing the context menu of the element.
On the top and the bottom of the table, locate the "plus" sign and click on it to add a new rule or column.
This will add a new rule to the bottom of the table.
Access the context menu by right clicking on the rule or column. Select the option where you wish to add a new rule or column.
This will add a new column to the right of the selected one.