Since Windows 8.1 libraries are not shown if you want to switch to another folder in MP3Tag.
Since Windows 8.1 libraries are not shown by default in windows-explorer. But you can bring them back by activating "Show librariess" in the navigating panel.
But this option doesn't bring them back in MP3TAG.
Other programs still show libraries when you select the open-menue or "store as"-menue item.