To make Lubuntu more usable inside a VirtualBox VM, you need some setup, like installing the Guest Additions.

  • Install prerequisites: 
    $ sudo apt-get install dkms

If you are using an old version of Lubuntu, such as 15.04, make sure the graphic card is VBoxVGA.

  • Insert Guest Additions CD image from Devices menu.
  • Open the Terminal and navigate to the mounted CD folder.
  • Install Guest Additions with: 
    $ sudo sh ./VBoxLinuxAdditions.run
  • Reboot

You can now share a folder between host and guest OS, creating a new shared folder from VM settings in VirtualBox.
If you have permission problems, add your user to vboxsf user with this command:

$ sudo adduser [your-username] vboxsf

Reference

lubuntu VirtualBox VM