While working on my final year project, I found that if you have installed Ubuntu as a virtual machine in your Virtualbox, you will find that the mouse integration does not work, even after you have installed the guest addition.
Mouse Integration is a awesome feature which helps you to move you mouse b/w guest and host operating systems without locking it. Nothing can beat the feeling of not having to search for your mouse when you switch OS and the cursor moving at different speeds.
(1) Start the Virtual Machine on which you want to work. First you have to make sure that have install Guest Additions. From the VirtualBox's menu go to Devices → Install Guest Additions... This will mount a virtual CD on your /media/cdrom. Normally this folder's window will show up. As root run the program VBoxLinuxAdditions.run. When the program completes reboot your VirtualBox.
(2) Open a terminal, type
sudo gedit /etc/X11/xorg.conf
(3) Add the following lines to the end of the file. Save and close
Section "InputDevice"Identifier "vboxmouse"Driver "vboxmouse"Option "CorePointer"Option "Device" "/dev/input/mice"EndSection
(4) Restart the computer. The mouse integration should work now. If you face any problem drop in a comment.