First, start up your Ubuntu virtual machine. Then, on the VirtualBox file menu, choose
Devices-->Insert Guest Addition CD Image.
Next, log into Ubuntu and start up terminal application by clicking the Unity icon and enter terminal. Click on the Terminal application.
On terminal, change your path to the inserted media. To do so, enter
$ cd /media/<user_name>/VBOXADDITIONS_5.0.6_103037/
where replace <user_name> with the Ubuntu username you have entered while installing Ubuntu. In fact, you can use the terminal's tab completion feature; for example, you can simply enter
$ cd /med <Tab> <Tab> <Tab>
which will automatically complete the path. Here, <Tab> refers to the Tab keystroke.
NOTE: do not enter $ symbol. This symbol indicates the terminal commands.
Look up files in this directory by typing
$ sudo ./VBoxLinuxAdditions.run
Once again, you can utilize the tab completion feature. Here, sudo commands lets you run the script with the root privilege, which is why it will prompt your Linux root password.
Wait patiently while it is installing the Guest Additions. When complete, it will resume to the terminal command prompt.
Eject the Guest Additions CD by entering
$ eject cdrom
For the installation to take effect, reboot your guest OS by entering
$ sudo reboot
or simply click on the right-most icon on the menu dock on the top of the screen and select down Shut Down and choose Restart.
Wait until Ubuntu reboots, and log back in. In order to set the resolution, enter displays after selecting the Unity icon on the dock. Set the display resolution as you desire.
You may have noticed that Ubuntu is running slower; this is because you have increased the display resolution, but your virtual machine's video memory was set too low. In order to increase the video memory, you will first need to shutdown Ubuntu.
$ sudo poweroff
When the virtual machine completely shuts down, go to the setting and choose Display tab and increase video memory as you desire. I recommend that you also go to the System tab and increase the number of processor option in order to boost the speed of the virtual machine. There are other things you can tweak around to increase the speed. Remember, however, that the more hardware resources you allocate to the virtual machine, the less hardware resources will be available for your host system.