VirtualBox - Can't see mounted ISO

It's a good idea to start studying GNU/Linux in VMs. And you have chosen a good hypervisor for Windows. VirtualBox is almost free/libre as well as beginner-friendly. In my practice, I'd also like to suggest beginners that they start with VirtualBox.

If the Guest Addition ISO file is included in the main binary file (released under GPL v2) but not in the Extension Pack (non-free), then there's no major freedom issue.

However, I never used the Guest Additions in practice. I just checked the Section 4.1 of the VirtualBox manual describing said Add-ons. On GNU/Linux guest operating systems, I don't really need the features provided by the Add-ons. In other words, I can easily find better alternatives. Let's talk about the features provided by the Addition pack:

1. Mouse pointer integration. Any modern GNU/Linux distribution should support this out-of-box.

2. Shared folders. Just install SSH server on the GNU/Linux guest operating system, and you should be able to share files securely. (You need to set the NIC to non-NAT mode, or else configure port forwarding. I set two NICs for my VM. One is set to NAT mode to access Internet, and the other is set to Host Only to share files with the physical host and/or other VMs.) Use free/libre software like FileZilla under your Windows host to access your SFTP server in the VM.

3. Better video support. Even with this addition, you can't expect to use your VM as a gaming console.

4. Seamless Window. X11 forwarding is way better. Just log on your VM using command "ssh -X" (or maybe "ssh -Y" if you use Cygwin on your Windows host) and you can project any window to your Windows host. (Be sure that you are running Xorg rather than Wayland on your GNU/Linux VM.)

5. Generic host/guest communication channels. Same as 2, SSH is way better. Just configure your virtual NIC correctly. Use free/libre software like PuTTY or Cygwin to log on your VM.

6. Time synchronization. Totally useless if you have Internet access.

7. Shared clipboard. This is a bug, not a feature.

8. Automated logins. This is a bug, not a feature.

@nadebula.1984

Thank you for your advice - I think I understand your points.

In any case, it's also relevant for my learning curve to know why the ISO is not showing up (as it should and as it did on other Debian-based distros).
If you could shed some light on that it would be great.

Just realized I didn't mention that I installed Trisquel-Mini.

Besides Trisquel Mini, I have now installed Trisquel Full and it automatically mounted VBoxGuestAdditions.iso as expected.
Presume no copyright issues are involved.

The problem resides with Trisquel Mini, which does not mount that package for some reason I don't realize.
That's why I'm calling for help.