Can't install Guest Additions - (Missing 'Devices' Menu on host)

When in seamless/fullscreen mode, you can hit Host + Home key to popup the toolbar dialog menu.

I know this post is old/may not have been a hotkey at the time, but figured for people visiting trying to find this (as it's high up on search results) it may be helpful!

Please update to the latest version of VirtualBox, version 4.3.26. The Devices menu will normally only be shown at the top of the virtual machine window when a virtual machine is running. You can also also use the keyboard shortcut to insert the Guest Additions CD. By default, to do this, hold down the Ctrl key on the right right of your keyboard, and hit the "D" key.

I had to hit HOST + C to disable stretched display (Right-Control C) and the Devices menu was shown on the Guest window NOT on the Virtual Box window.