You need several new constants in the include file to identify the controls of the FileOpen dialog box. Add the following statements:
#define IDC_FILENAME 400
#define IDC_EDIT 401
#define IDC_FILES 402
#define IDC_PATH 403
#define IDC_LISTBOX 404
Although you may choose any integer for a control ID, the ID for each control in a given dialog box must be unique. By convention, a predefined ID, such as IDOK or IDCANCEL, is less than 100, so any number greater than 100 can be used for other controls.