I installed Lubuntu 12.04 via the mini.iso on my ancient Thinkpad X40, and had to install an additional command line tool in order to follow the instructions to add the PPA for fake-pae.
Depending on the specific distribution, you may need to install python-software-properties or software-properties-common in order to have the apt-add-repository command available at the command line.
On my 12.04 install, I needed to run:
sudo apt-get install python-software-properties
Apparently on later versions like 12.10, the package is:
sudo apt-get install software-properties-common
I wanted to post this as a follow-up in the thread, but something about the ubuntuforums configuration prevents me from doing so.