The reason might be because the IPython module is not in your PYTHONPATH.
If you donwload IPython and then do python setup.py install
The setup doesn't add the module IPython to your python path. You might want to add it to your PYTHONPATH manually. It should work after you do :
export PYTHONPATH=/pathtoIPython:$PYTHONPATH
Add this line in your .bashrc or .profile to make it permanent.