Dmitry - if you wish to use the same code in your GUI, then instantiate the cameraHandle in the _OpeningFcn of the GUI, save it to the handles structure, and then reference it in your pushbutton code. Try the following
function myGui_OpeningFcn(hObject, eventdata, handles, varargin)
% Choose default command line output for untitled3
handles.output = hObject;
% create the COM object
handles.cameraHandle = actxserver('maxim.ccdcamera');
% Update handles structure
guidata(hObject, handles);
And then in your push button callback, just do
function buttonExpose_Callback(hObject, eventdata, handles)
exposureTime = 42;
invoke(handles.cameraHandle,'Expose',exposureTime);
Note that any callback that receives the handles structure as an input parameter, can reference the cameraHandles via
handles.cameraHandle
