You should probably consider stripping all whitespace from the user input.
This looks to me like the value in the text field is not equal to either of your branches, meaning m is not defined. You are also using == to compare char arrays, instead of strcmp (which would also handle strings and will handle arrays of dissimilar size).
switch app.text1.Value
case 'm'
m=app.num1.Value;
case 'a'
a=app.num1.Value;
otherwise
%make sure your user gets an error here, or set default values
end
Another possibility is to use a dropdown menu or radio buttons to allow the user to select variables.
