Gabriel - to be clear, are you expecting the user to type in RETURN or DELETE or press the return or delete keys on the keyboard? The following line of code
key = get(fig, 'CurrentCharacter');
will just return the character of the pressed key. For a button like return, key will be an empty character.
If you want to capture the input from the keyboard, then I would change your callback to
function keyRead(src, event)
key = event.Key;
fprintf('The key being pressed is: %s\n', key);
end
By the way, I'm not sure why you have the uiwait or are using the f instead of fig. What is the intent of the former, and is the latter a typo?
