The control has a color bar showing the active color table and the associated high and low data values in the units the display is made in. As the mouse pointer is moved over the color bar, the value at a particular color is shown. Click on the color bar to start the Color Table Editor. Or click on the button that displays the name of the color table to show a popup menu that allows you to change the range, select other color tables, etc.
Click on the 
button
or enter a value in the field and press return to set the value of the
isosurface.