I believe that recent versions of the provided VM are broken.  I have tried a few different downloads and a few different versions of VirtualBox and I get the same failed result.  It seems that something is wrong with how they are currently being built.  I have already filed a ticket:

5262https://trac.openmodelica.org/OpenModelica/ticket/5262


Guest additions is not really a OpenModelica issue, but is instead an issue with VirtualBox.  But, perhaps this will help:
VBox Additions

Usually, I use a native build of OpenModelica on my Mac, but that currently fails as well. 

Good that you have found the recent build is broken and made a ticket!

Concerning installing guest additions to my older still working VM with OpenModelica I have a few questions.

Exactly what operating system is it?
- in the text here https://openmodelica.org/download/virtual-machine
   you say it is 32-bit, but in the download section you can choose both 32-bit and 64-bit and I chose 64-bit
   does this refer to the same thing?
- understand it is Xubuntu you use that is a lightweight Ubuntu, but what version is it?
  are you sure that your link with recommendations for Ubuntu on guest additions also hold for Xubuntu?

I also want to say that I have used VBox over about 10 years mainly with guest Windows but also Ubuntu and also tried Fedora
but never hade problems with mounting the virtual disc with guest additions. Occasionally there have been smaller problems
with actually making guest additions to work, but that is easily overcome. But with VM OpenModelica I cannot even mount the disk.

I don't know what OS the newer OpenModelica VMs are built on.  I have an older one around, and that one does use Ubuntu.  I just tried to mount the Guest CD and got an error that it could not mount because the VM doesn't have an optical disk.  So, you probably need to add a virtual CD, and perhaps some drivers.  Or you can add the guest additions, as suggested above, and suggested here:  Title from the command line in Ubuntu.

Hi,

Thanks for the advice and links!
I have played around with a clone of my working VM with OpenModelica and found the following
detailed recipe DOES work (for me at least):

First I did the new small upgrade that I got today.

Then the following:

1. Before starting the VM open up settings/storage and for the Controller select add Optical  Drive and choose Leave empty
2. Start VM
3. Open terminal
    1. Cd /media
    2. Sudo apt-get install VirtualBox-guest-additions-iso
    3. Sudp apt-get install VirtualBox-guest-utils
    4. Shut down VM
4. Open
    1. VM settings/Shared folders and choose folder you want to share
    2. VM settings/Storage and for the Controller right-click and choose remove attachment
5. Start VM
6. Now the folders are shared!

The other important functionality that guest-additions should is to make it possible to resize
the window and make it larger. This did still NOT work.

I made one small observation that might be interesting.
In the VM (Ubuntu-Xenial) I opened up settings/Display and see the resolution 1024x768
When I change the size of the window nothing happens to the shown resolution and
very logical since making the window largerer only introduce white borders.

I happen to have another VM with standard Ubuntu LTS 16.04 and started that and looked at the
same setting/Dispaly - when I change the size of the window then the resolution do change.

I would be very glad if the window could be made to be re-sized!
OMEdit requires a quite large screen to be comfortable.

Appreciate further advice!

Hi again,

I discuss the issue on VirtualBox forum.

They ask me why under general settings for the VM the Version
of Linux is stated as 2.6 /3.x /4.x (64-bit) and not just Ubuntu?
Someone who know?

They also think the procedure I used (after recomendationd here) to install "guest additions"
described above is not proper - it only gives some third party version of guest additions, see

https://forums.virtualbox.org/viewtopic … mp;t=91300

Would appreciate to get VM OpenModelica to work better!

It appears that the setting of linux version probably doesn't matter:
VBox Version  It appears to just be a convenient way to do a bunch of other settings.  Hopefully, those are already correct. 
You probably also need to upgrade your version of Linux to upgrade the vbox add ons.

I wait with testing change of general settings for the VBox. I have a discussion running in VirtualBox forum on the subject. They wonder what role "Vagrant" has - which they find in my log file. Now I also see that OpenModelica organisation has made a new VM dated 2019-01-16 that hopefully works better than the one released in December. But I see here here two files with Vagrant in their fila names. In the readme documentation there are not mentioned. Do I need to download them also? Why are they there? Is perhaps the VM made in Vagrant software originally?

Yes, the VM is generated by vagrant because it was convenient to script using it. But it seems the image has changed such that only serialport communication is used. When I get a little more time I will setup new VM builds. Until then, just take any Ubuntu image and follow the Linux installation instructions