enabling virtual machine Guest Additions on a Mac host and Mac guest
I'll quote a previous answer of mine:

From Virtualbox Manual : Chapter 14:

14.2. Known Issues
Mac OS X guests:
VirtualBox does not provide Guest Additions for Mac OS X at this time

[snip]
From the content of this, I wouldn't hold out for such support in VirtualBox any time soon.

The situation really has not changed since then.
There is some discussion of a workaround involving the built in OS X sharing functionality and configuring your network setup to work, but nothing well defined.