I am also trying to use the GRETNA Toolbox to do the voxel-based degree. The Matlab errors that I have are:

Error using gretna_GUI_PreprocessInterface>RefreshStatus (line 2563)

Error in gretna_GUI_PreprocessInterface>RunEvent (line 2513)
RefreshStatus(handles.AliasList, handles.InputListbox, handles.PipelineLog);
Error in gretna_GUI_PreprocessInterface>RunButton_Callback (line 2640)
RunEvent(hObject , handles);
Error in gui_mainfcn (line 96)
feval(varargin{:});
Error in gretna_GUI_PreprocessInterface (line 42)
gui_mainfcn(gui_State, varargin{:});
Error in @(hObject,eventdata)gretna_GUI_PreprocessInterface('RunButton_Callback',hObject,eventdata,guidata(hObject))
Error while evaluating uicontrol Callback
Undefined function 'CalInterface' for input arguments of type 'char'.
Error in gretna_GUI_PreprocessInterface>PopupMenu_Callback (line 2744)
CalInterface('Connect');
Error in gui_mainfcn (line 96)
feval(varargin{:});
Error in gretna_GUI_PreprocessInterface (line 42)
gui_mainfcn(gui_State, varargin{:});
Error in @(hObject,eventdata)gretna_GUI_PreprocessInterface('PopupMenu_Callback',hObject,eventdata,guidata(hObject))
Error while evaluating uicontrol Callback

I tried to run this toolbox on Matlab 2015 and Matlab 2013, and I got the same errors.
Any help would be highly appreciated.

Gaelle