Open a terminal and execute:
sudo apt-get install virtualbox
which should work.
Once you have installed it you probably want to install guest additions in the guest OS.
Also you probably want the extension pack which you can get from here:
Make sure you get the correct version.
As of Xenial 16.04, you can also get the extension pack from the Canonical repositories. Go to the terminal and execute:
sudo apt install virtualbox-ext-pack
Therefore, as of Xenial 16.04, you should be able to get virtualbox and the extension pack by typing the following:
sudo apt install virtualbox virtualbox-ext-pack
Following the instructions on your screen.
To install the user interface to manage your virtual boxes use this command
sudo apt install virtualbox-qt
Pepper Flash is now in Ubuntu repositories (14.04). Install it like:
sudo apt-get install pepperflashplugin-nonfree
sudo update-pepperflashplugin-nonfree --install
You can type in the terminal: