Hi,
please try the latest development version from here: https://builds.jabref.org/master/
Remember to make a backup of your bib file before,
content selectors in the entry editor are working through the autocomplete dialog (ctlr+ space) to open the popup.
But the manage content selector dialog is broken, I filed a bug for this.
The dialog is still a dialog with the old GUI technology and needs to be converted. It’s one of the few dialogs that have not yet been converted.
Regards
Christoph