I had this very same thing when I first tried running a Zorin OS on Virtualbox.
But yes, since Zorin is built off of Ubuntu, it can be treated as Ubuntu in many things. If you are having some other issues that need troubleshooting, you can search for that issue under Ubuntu 18.04 (For Zorin OS 15) to find applicable tips. This does not necessarily apply to Zorin OS Specific applications, though.
To me, this sounds as though you are seeing the desktop without a background image, with a nonfunctional panel at the bottom. I could be wrong- and someone who is better at using Virtualbox than I am may know what that is and of a very easy fix.
But I have barely ever used VirtualBox and it has been a long time since I have.
VirtualBox does have a forum though:
Where more knowledgeable people may hang out.
In the meantime, I wouldn't want it known I casually foisted you off on someone else so...
Let's check your Installation Method, in case there is something in it that may solve the issue sooner.
Here is a guide assuming Ubuntu 18.04. Using that, we can check your steps on Zorin OS 15.
Step 1 is covered. Step two includes providing a name for the OS. You can name it Zorin, though the guide named it Ubuntu since that is what the guide is for. But really, you can name it whatever you want. You can name it ZorinMcZorinface if you want to.
Memory size- I would suggest greater than 4gig.
Version: Ubuntu 64bit
File location can be Zorin
Please update this thread with results.