I installed IPython to my Mac by MacPorts. I can run it by $ IPython, by not as a login shell.
I have tried to find IPython's directory, which I could then put to my terminal's settings, unsuccessfully by
locate /*IPython
I did not manage to find it even by whereis.
How can you set IPython the default login shell?
locate is not a live database so I'm not surprised that it can't find IPython assuming you just installed it
How did you install, IPython? There must be documentation with the package about the install location?
If you have installed it using macports then look in your macports directory (usually /opt/)
If it is /opt/ then look in /opt/local/bin for your python installation