I want to install Guest Additions in a virtual Windows 7 running inside Ubuntu. The VirtualBox instructions are:
In the "Devices" menu in the virtual machine's menu bar, VirtualBox
has a handy menu item named "Install guest additions", which mounts
the Guest Additions ISO file inside your virtual machine. A Windows
guest should then automatically start the Guest Additions installer,
which installs the Guest Additions into your Windows guest.
How do I get to the "Devices" menu in the virtual machine's menu bar?
You have Scale Mode enabled. Disable it (Right-CTRL + C) and you will see the Devices Menu for your VM. You can then install the Guest Additions