How do I copy and paste text from Mac Host to Ubuntu Guest in VirtualBox?
There are no guest additions for Mac because macOS is not supported to be run as a guest in a virtual machine on a system other than macOS. In your setup, Ubuntu is the guest system and macOS is the host system.
The guest additions are to be installed in the guest system. Just start your Ubuntu virtual machine and select Devices -> Insert Guest Additions CD Image from the VirtualBox command menu to install the guest additions into Ubuntu. After that, copy and paste should work, provided that you activated it in the settings as you described.