Here are the instructions to install HHVM on Ubuntu 12.04.

sudo add-apt-repository ppa:mapnik/boost
wget -O - http://dl.hhvm.com/conf/hhvm.gpg.key | sudo apt-key add -
echo deb http://dl.hhvm.com/ubuntu precise main | sudo tee /etc/apt/sources.list.d/hhvm.list
sudo apt-get update
sudo apt-get install hhvm

To test if your HHVM is installed, save the following into a file called test.php. Notice there is NO closing tag for hh.

<?hh
  print "hellon";

Then run hhvm test.php, you should see the hello string gets printed out.