Hey... notice that the AppDesigner code you're editing is a class. And then, you can't define a class directly within the code of another one.
Another point: it's important that you try to separate the front-end from the back-end, so it might be important for you to create classes outside of AppDesigner - using classdef - and create instances of it inside the app. To not lose the reference, you create a property in AppDesigner and store a handle to your object.
Finnaly, an app has its own workspace - its properties! If you're reading an external file to then do post-processing and plots, I suggest you store everything in the app's properties. If this is the case, you can open external file directily your app, using uigetfiles and related functions (readtable, fileread and so on).
% Outside AppDesigner (save as .m file)
classdef myClass
properties
classProperty
end
end
% Inside AppDesigner (.mlapp file)
properties (Access = private)
myObj = myClass
end
