Hi, i am going through the same thing too. iv scoured the net for a solution but its looking bleak right now. i found this site which says
"Attention: Android 4.3 has changed the kernel so that loading driver modules is disabled. Nexus 7 Old still has modem drivers enabled in the kernel, but Nexus 7 New (Wifi) and Nexus 10 don't. Since there is no more way to add drivers, USB modems cannot be used with these devices anymore. Blame Google for this."
can someone confirm this or say otherwise. am really bumd that they would leave this feature to use an external modem out. when old nameless tablets can do it. ahhhh