I know this has been asked before and I've searched the forum, I have followed one option of changing the port to 3389, this I've done, another is to install Guest Additions.....but during the install Guest Additions were downloaded according to the install log...
2017-07-18 12:10:30 (7.91 MB/s) - '/tmp/Oracle_VM_VirtualBox_Extension_Pack-5.1.8.JZx1YklNtX.vbox-extpack' saved [19475839/19475839]
WARNING: The character device /dev/vboxdrv does not exist.
Please install the virtualbox-dkms package and the appropriate
headers, most likely linux-headers-amd64.
You will not be able to start VMs until this problem is fixed.
0%...10%...20%...30%...40%...50%...60%...70%...80%...90%...100%
Successfully installed "Oracle VM VirtualBox Extension Pack".
if the above is correct how do I apply the Extension Pack to the installed VM, reading the VB user manual it tells you mount the iso as a virtual cd/dvd which makes it more confusing...and I'm guessing there is a simple answer 
