I believe you got stuck on a configuration. 2sxc 8.x has many new features which you can enable/disable on such a relationship. I believe right now you have set it to
- don't allow multiple items
- don't allow remove
so you have one item mapped, but you can't remove it, and you don't see the dropdown because you didn't adding more items :).
So just go to the entity-field settings and configure it.
Warning: there is a special case where you may have an old entity-field, and never configured which input-type you want (because there used to be only one). In this case, you want to first set which input-type you want to use (Entity-default), then you'll get the remaining fields to set:

and then
