If you entered concentrations, Prism can transform those values. Also check the option box to create a new graph. If you enter a concentration of zero and then transform to logarithms, the result will be blank. Beware of entering the zero value as a value much lower than two log units below your lowest concentration. When fitting a sigmoid curve, Prism sets the initial estimated value of the IC 50 to be halfway between the lowest and highest X values.
Prism offers two equations.
If you have plenty of data points, choose the equation with variable slope. If data are scanty, use the standard slope. Ideally, the top part of the curve is defined by at least several data points. In this case, Prism will be able to fit the top plateau of the curve. If the top plateau is not well defined by data, then you'll need to make the top plateau be a constant based on controls.