Solved it:
function readData_Callback(hObject, eventdata, handles)
% hObject handle to readData (see GCBO)
% eventdata reserved - to be defined in a future version of MATLAB
% handles structure with handles and user data (see GUIDATA)
handles.output = hObject;
[file,path] = uigetfile({'*.xls;*.xlsx','Excel Files'},' Select data file')
filename = strcat(path,file);
set(handles.readData);
data = xlsread(filename);
data(:,[1 7])
set(handles.uitableData, 'Data',data);