When a new calculator is created, it is only given one row. To add a row, put the cursor in the last row and hit the "Return" key on the keyboard.
If there are several rows, in order to execute a command in one of them you have to designate which one. There is space to the left of each row, immediately to the left of the "Name" box; move the mouse-point to the left of the row you want to execute, and click the mouse-button. A pointer will appear, indicating that this row is the next one to be executed. Now when you press the Do It button, the calculator will execute the indicated row.