How to fix low resolution in Oracle´s VirtualBox

At first install Extension Pack

Open Extension Pack using VirtualBox

Run the operating system and insert Guest Additions CD image via VirtualBox´s bar

Devices -> insert Guest Additions CD image

Now ”the image” should see as an CD image

Copy CD´s contents into your $HOME directory and open Terminal in that directory and run

sudo sh

The the Additions are installed, low resolution should be fixed.