According to http://www.listary.com/total-commander-integration I'm supposed to press Win+O to switch to TC in an "Open file" dialog. Nothing happens, instead I just enter "O". Then I found a post saying that Win+O was removed. But Quick Switch doesn't work either. Pressing Ctrl+G does nothing.
For what it's worth, I'm using TC 9.0a rc1 on Windows 10.