you are installing the product using the graphical installer, to do so the OS will need to display the UI, here linux is telling you that it cannot find the DISPLAY variable configured .
This is not an issue with the WebCenter Sites installation process in itself, it's more of a unix OS configuration thing .
As Pietro mentioned this is not a product issue, with your current setup you cannot access the UI for installing the product as you are not being able to use AWT.
However you can install the product in silent mode which is what you asked for if I'm not wrong. Here is the link with the steps to do so:
Hope it helps.
Okay, I also had this problem when I installed JSK in my macbook air.
You have add one parameter in your bash_profile.
Open terminal -> type the command without qoutes -> "vi ~/.bash_profile" -> add the following line -> export DISPLAY=":0"
Save it and then run. It worked for me, hopefully works for you too.