wspeed=(nwind*nwind+ewind*ewind)^.5
where the "^" operator means "raise to the power". In this case, we need the square root of the sum of the squares of each wind component.
When you are done entering the formula (or formulas, separated by a comma), touch the Enter or Return key. The data are relisted, with the new field name and values added as a new column of data to the right of the existing table.