You’ll need to install numpy, scipy and matplotlib to get pylab. In ubuntu you can install them with this command:
sudo apt-get install python-numpy python-scipy python-matplotlib
If you installed python from source you will need to install these packages through pip. Note that you may have to install other dependencies to do this, as well as install numpy before the other two.
That said, I would recommend using the version of python in the repositories as I think it is up to date with the current version of python (2.7.3).