i had to reinstall windows and opera too. after doing so i had this problem. what you have to do after you install pop up blocker extension is to disable it then re-enable it and click on the cube thing in the tool bar and click that 3 dot thing and a drop down menu opens then click "pin" and it will appear on your tool bar.
what was happening was when you first install that or any extension, by default it was not pinned to tool bar or even the cube when you click it. how to pin and unpin things to your tool bar is the problem here. in my opinion its time for new developers at opera because it will take them years before they fix this or even think its not a good idea. Time for some new talent opera!