Run following Commands to Run VBoxLinuxAdditions.
Note :- Guest Addition is used to Add-on Extra Features to your Virtual Machine.
1} Change Directory to Desktop.
2} List all the items present on Desktop.
3} After Doing ls you will see VBoxLinuxAddition.run. Give all the Permission to it using following command.
chmod -x VBoxLinuxAdditions.run
4} Run VBoxLinuxAddition.run.
5} Reboot your Virtual Machine/System.
Note :- If you are unable to find it on Desktop then Search it on the Optical Drive/Storage Media. You may find it on your Storage Media.
If Still have any problems running Guest Addition or to make Linux Full Screen.Mention your Errors in Comment Section.