Virtual machine, mouse cursor only works within the box

Right CTRL key to release the mouse and keyboard.
Documentation on the host key is here. <-- Credit to Takkat on the link

Install guest additions for transitions without manually releasing the keyboard/mouse.

To get your mouse cursor to work, you simply need to press the Ctrl key on the right side of the keyboard. If you don't want to keep pressing Ctrl to release the mouse cursor every time, simply install the VirtualBox Guest Additions by clicking on Devices > Install Guest Additions. When you do this, you may need to mount the virtual cd-rom drive by using the following command:

$ sudo mount /dev/scd0 /mnt/cdrom

Then run the shell script to install the Guest Additions.