virtualbox.org

hi,
i've installed VirtualBox on a windows 10 laptop, and I've started a Lubuntu guest OS.
My problem is when I click on devices>"insert guest additions CD image": it opens a window called "removable medium is inserted" with the option "open in file manager". When I click on "ok", it opens the folder VBOXADDITIONS_5.0.14_105127 and here I don't know what to click on.
Could anyone help me?
Thank you

That is a CD disk, not a folder.

Detailed instructions on how to install the Guest Additions into a Linux guest is given in the user manual.