I think I figure out what might be happen.
you have to change your mode switch to a button, so DCS can recognize it as a modifier.
first you have to open the logitech program software, then choose throttle program, and click in view so you can see the modes buttons in blue color. click the mode 2, for instance, and choose no buttons. save your profile and apply it to your hotas. Now back to DCS it will recognize it as a button and you can add it as a modifier.
Hope this helps!
Cheers