The problem is that my Windows guest VM comes up with no outside window around it. It's in this window (see Conquering Sun VirtualBox for Dummies like Me) that the Devices menu appears to let you install the guest additions.

The moral of this story is not to go horsing around optimizing your guest OS window at least until you've at least installed the Guest Additions. The second time I installed, I was careful and did not have to do anything other than grab the menu at the top of the guest window Devices -> Install Guest Additions....

Instead, I had to download the guest additions ISO (see here) and then go through my (unlaunched) VM's Settings -> Storage > Controller: IDE, etc. to mount the ISO (that I downloaded) as a CD.

Then, I launched the Windows VM and opened the (now showing CD) in Windows File Explorer. It would not AUTORUN. Duh. And double-clicking AUTORUN only opend the file in NotePad. Double duh. So, to get it to install, I double-clicked VBoxWindowAdditions-amd64. It took a while to launch.

What's the mess here? Somewhere along the way, I either naively got rid of the outside VB window around my launched Windows guest VM or VM just sucks. I googled heavily, found others complaining also and no one offered a solution except to repeat "install guest additions by pressing HOST + D". One respondant even said, "...read the documentation. I can't improve on it." I was never able to get the VirtualBox window (parent) of my Windows 7 guest VM back which would have made installing guest additions easier.

As soon as I got the guest additions installed on Windows, however, I immediately saw what I was looking for: a working, shared-folder relationship with my Lucid host. The first illustration is the VM settings for my Windows guest showing where I thought I'd be sharing a mount point. The second is what I see in Windows File Explorer, i.e.: an ISO I had intended to share (for purposes unrelated to this discussion). The third shows you where this is on my Lucid host.

I tried installing Ubuntu Precise Server on Linux Mint 13 under VirtualBox. A sore problem with getting help on this topic is understanding when respondees are talking about something to do or happening on the guest OS as compared to the OS hosting VirtualBox.

This went well, but I could not:

1. Install Guest Additions.
2. Use Ctrl key on guest.

Per https://forums.virtualbox.org/viewtopic.php?f=3&t=15679, I did this:

1. apt-get install dkms build-essential linux-headers-generic
2. thinking that this would allow the (Guest window) Devices -> Install Guest Additions... to work. It did not.

Ultimately, I was able to install the Guest Additions, but I did not note how. As I remember, I used the advanced packaging tool.

The best way to avoid trouble is not to jack with any of the settings, but just dismiss the alerts each time you start up.

How to mount what's in the host's CD, DVD, Bluray, etc. drive in the running virtual machine (VM):

1. Open VirtualBox manager dialog (main window that lists all the VMs) if not open.

2. Select the running VM.

   

3. Click Settings (will be for this VM), then Storage.

   

4. Click Controller: IDE → Empty; you should see something like this:

   

5. Click to mark Live CD/DVD if not checked.

6. Click disk icon (choose a virtual disk...) and choose the drive in the host running VirtualBox you wish to mount in the VM.

   

7. Click OK.

At this point, you should hear the drive begin to whir, then see it mount on the VM's desktop.

To mount the host's (shared) folder (VirtualBox Guest Additions must be present, installed and mounted). The mounted folder was designated in the VirtualBox GUI Settings → Shared Folders configuration. For instance, maybe you're running VirtualBox on Windows and sharing your Documents folder. Here's how to mount that on a headless, Linux server. These commands are all run as root.

1. Create a mount point under /media.

   # cd /media
   # mkdir sf_Documents
   # chmod 770 Documents
   

2. Fix up the vboxadd user to belong to group vboxsf:

   # adduser vboxadd vboxsf
   # groups vboxadd
   vboxadd : daemon vboxsf
   

3. Mount the shared volume:

   # mount -t vboxsf Documents /media/sf_Documents
   /sbin/mount.vboxsf: mounting failed with the error: Protocol error
   

   This seems always to happen when the guest is headless. Fighting it for hours results in it working, but there appears no rhyme or reason for it to have failed in the first place and to begin working in the second. The last time I got it to work, this was the configuration:

   VirtualBox → Settings → Shared Folders → Name/Folder:
   dev/C:\Users\rbateman\dev
   # mount -t vboxsf dev /media/sf_dev
   

It's relevant to understand what one's networking options are in a VM depending upon the network type chosen in Settings → Network → Attached to:. See 6.2. Introduction to networking modes.

To read what's in the table's left column, take VM ⇆ Host; this means, "from VM to host or host to VM." VM → Internet means, "from VM to Internet (but not from Internet to VM)."

Bridged networking in a VirtualBox guest means more or less that the VM behaves on the network as if a computer host separate from the one on which it's a guest. The other types come with various limitations that make them not behave as an independent, separate host.