I was looking for a screenshot on the forums to show you the icon, and one of the most recent is from you 
Starting from the top right, read it to the left, you have:
- user
- an icon that looks like a hard drive
- another icon made of one square in front of another, that’s the one you’re looking for
So, right now, you’re able to copy-paste to and from dom0, but looking a solution for the following problem?
What’s in your RPC policies about the clipboard? In dom0, open Qubes Global Config then, select the Clipboard tab. Is the qubes-qrexec-policy-daemon service still running?