That code you gave will give you a single item (the last one) in the popup. Do not use a for loop. The items in a popup or listbox are not entered in one at a time like you tried, they are done all at once with a single call to set the string property. What you did was to set the entire list to just a single item, and you did it 3 times (once on each iteration of your for loop). Since there is only one item in there, and the value is now 3 (which is more than 1), the popup will not display.
To fix, send in all the items in one shot, then set the value.
var1 = {'Ex1', 'Ex2', 'Ex3'}
set(handles.popupmenu1, 'String', var1); % Send in all 3 items at once.
set(handles.popupmenu1, 'Value', length(var1)); % Set what item is selected (the last one).
