On Ubuntu 18.10, the default size of the window opened by the plasma-nm is too small for the controls (the "+" and "-" signs at the bottom of the left list of networks) to show. You have to scroll down to see them.

I was confused enough to have to search on google how to add a VPN connection.

My suggestion would be to keep the buttons "floating" (i.e. showing without having to scroll), or to move them elsewhere (above the search bar ?).