1.Go to virtualbox linux download page and download the deb package file for Ubuntu 11.04 (Natty Narwhal) (32-bit or 64-bit).
2.Double click the .deb package file or open with ubuntu software center and click the install button.
3.Type your ubuntu password if needed and wait until the installation is complete.
4.To open virtualbox, click Applications icon from unity launcher. And type virtualbox in the search applications field and click Oracle VM virtualbox.