Add a new column by either double clicking on the empty column header area to the right of the last column in your data table, or by using the Column pull down menu and selecting "new Column".
Then, right mouse click on the new columns header and select formula. The formula editor will open up. In the column display area in the formula editor, double click on the column you want to be the numerator in your formula. Then click on the divide by symbol, then click on the column that you want to be the denominator. Finally, click on "Apply".
Jim