It's the other way around. The app.SelectInputDataDropDown.Value expression is the user's selection, so to assign that to some variable you want, you have to put it on the right hand side of the equation, like
temperatureInCelsius = app.SelectInputDataDropDown.Value;
temperatureInFahrenheit = (9/5) * temperatureInCelsius + 32;
or perhaps if you want the contents of the list rather than it's index, you can do
allItems = app.SelectInputDataDropDown.String;
selectedIndex = app.SelectInputDataDropDown.Value;
temperatureInCelsius = str2double(allItems{selectedIndex});
temperatureInFahrenheit = (9/5) * temperatureInCelsius + 32;
