I have a 64-bit installation and machine with VM enabled in the bios so I choose this installation:
After the download, open it with the software center.
If the Ubuntu Software center says you need to update the package for any reason do this first then do the installation.
Once the installation is complete, you should be able to confirm by simply clicking the Windows Key or if you are running Unity , you should see the "Oracle VM Virtualbox" . Once you click on that icon it should open the Oracle VM Virtualbox Manager without any issue and allow you to play with your Virtual box VMs/ Here are a couple of websites that have some prebuilt Virtual Boxes for you to play with, the first one brags of having over 240,000 prebuilt Virtual Box Images. Enjoy...
If you have any issues please let me know and I wkll gladly see if I can give oyu a hand...