So, let's get started. First, you need to get VirtualBox from here, and a image of Ubuntu from here (please choose your country). Install VirtualBox like any other application for Mac OS X, and start it. Let the image of Ubuntu like it is.
Now, on VirtualBox, do the following:
- click "New"
- click "Next"
- give it a name and at operating system select "Linux", and at version select "Ubuntu"
- select an amount of memory to allocate to Ubuntu (do not give it more than your current free RAM memory; 512 MB are really enough if you don't have more) and click "Next"
- be sure that "Create new hard disk" is selected, and click "Next"
- click "Next"
- check "Dynamically expended storage" and click "Next"
- here just select the size for Ubuntu, and click "Next"
- now click "Finish"
- click "Start"
- select "OK"
- when the window appears, click "Next"
- now, at the Media Type select "CD/DVD-ROM Device", and at Media Source click on the folder from the right, click "Add", and select the recently downloaded image of Ubuntu. Select it and click "Select"
- click "Next"
- click "Finish"
After the installation of Ubuntu is completed, give it the requested reboot. Don't worry if Ubuntu moves hard, it is normal at this moment. Now, on the window of Ubuntu, click on "Devices" (the button is up, on the silver bar), and select "Install Guest Additions". On the window that appeared, select just to open the folder. Now, drag the file named "VBoxLinuxAdditions-x86.run" on the desktop of Ubuntu. Continue by doing the following:
- on Ubuntu, go to "Applications" --> "Accessories" --> "Terminal"
- here, on Terminal type: "cd Desktop" (without quotes)
- type in Terminal: "sudo sh VBoxLinuxAdditions-x86.run"
- now you have to enter your password (that one you have entered when you installed Ubuntu; type it, and even you don't see anything, it is there; tap enter when done, or if you haven't entered a password, just tap enter)
- wait for a few minutes
- when installation is completed (last line is like the line you saw at the start), close the Terminal
- type "sudo gedit /etc/X11/xorg.conf"
- enter again the password you have entered during installation, and tap enter
- on the window that has appeared, modify where it says "Modes" with the resolution of your monitor (on my MacBook White 13,3" it is 1280x800)
- save that file and close it
- restart the operating system
Now you can change many settings in VirtualBox (while Ubuntu is shut down), as the video memory allocated to Ubuntu. Be sure not to give it too much, as the operating systems are sharing their resources. For Desktop Effects, 3D acceleration should be enabled, but on my Ubuntu (10.04 Alpha 3), the mouse disappears when the 3D acceleration is on.
So, that's all. Congratulations if you succeed. Your new Ubuntu installation is completed. Enjoy!
P.S: This works also for the installation of Ubuntu under Windows, because all you need to to is 100% the same.