Problem is solved:
If you enter items data it's stored as char. So therfor you have to cast from char to double or compare a string.
In my solution I cast a double out of char and then the switch-case works fine:
function changeView(app, event)
chosenView = str2double(app.DropDown.Value);
view(app.UIAxes, [0 90]);
app.UIAxes.View = [30 30];
set(app.UIAxes, 'View', [90 0]);
All three cases can be used to change the view!