Reference Functions Using Standalone Executable

I created a GUI that uses a function file to create plots, diagnostics, etc. The program was written to accept any type of function, which is the key to its usefulness. I created a standalone executable of the GUI which works great, but when I import my function and put it to use an error shows up (in the DOS window):

    ??? Error while evaluating uicontrol Callback
    ??? Undefined function or method 'fun' for input arguements of type 'double'.
    Error in ==> gui>my_sub_fcn at 1185

I know that functions are included in the compile stage, but this program is intended to use a dynamic function definition. Is it possible to achieve what I am trying to do?

Thank you very much for your help!

Richard