After installing Ubuntu 15.04 inside virtualbox, you need to install the guest additions to get better hardware functionality inside it. These include full screen resolution, mouse wheel and pointer integration, copy paste from host os (shared clipboard), shared folders etc.
This tutorial shows you how to install VirtualBox guest additions in Ubuntu. The same method works for similar distros like Xubuntu, Kubuntu, Lubuntu and other Ubuntu-based distros.
Virtualbox guest additions are compiled for the target system, so it needs the necessary kernel headers and related programs. Install the following 2 packages.
sudo apt-get install build-essential module-assistant
sudo m-a prepare
Go to the VBOX Addition media,
Then run ./VBoxLinuxAdditions.run