Undefined function or variable when calling external function in an app

I have a trouble when creating an app. There is an exeternal function which I wish to call it from a callback function in an app. The subfunction is put in the exeternal function and this subfunction is only used in the external function (the callback only provide variables). However, matlab cannot recognize the subfunction. How can I solve the problem?

[Sahithi Kanumarlapudi]

Do you have that external function in the search path?