Excellent guide! I did have problems with adjusting the display like others but after following the more detailed instructions of part II from the following link, I was able to resolve the issue.
It pointed out the importance of:
***Noting your Virtual Machine Name for Mac OS X and quitting VirtualBox. It is
the name displayed in the Left Pane of the VirtualBox window.***
I had VirtualBox and the VM still open thinking the cmd needed to connect to it (like a server, as referenced in the error). Then, I was using the specific name of the Mac as said under System Preferences, Sharing whereas you need to use the name that is stated in the start up window in VirtualBox (as said above, on the left side).
I hope this helps!