You cannot install a printer in the usual way on a Chromebook (without installing a Linux distro + CUPS etc), it can only print to Google Cloud Print connected printers. Usually you would need a computer with Google Chrome to do this, but if you have a headless Ubuntu print server there are other options. The simplest way I have found is by installing cloudprint from the Ubuntu repositories. (I assume it is also available in other distros repositories too.)
sudo apt-get install cloudprint
Then once that's finished run
It will then prompt for your Google account name (usually your Gmail email address) and password, then in a moment it will hopefully show up like so:
Then you can manage your printer(s) from the Google Cloud Print website and now I can print from my Chromebook or anywhere from Chrome or Android. I have a networked HP 2600n and have added it to my Ubuntu Server, which is on 24/7.