The Concentration field is comprised of two parts: the left field is the number part of the concentration and the right dropdown is the concentration units. To enter a concentration, put the number part in the left box and choose the units from the right box. You can either click the down arrow to open the dropdown or type the first letter of the unit you want to use until the one you expect appears in the window. You can also type the units with the number part and it will transfer the units to the units box for you.