First you must download the source code for omnifit. The most up to date development version of omnifit can be cloned from the github repository.
Once you have downloaded and extracted the Omnifit source files, you can build it by invoking the command
python setup.py build
in the directory containing setup.py. After omnifit is finished building, you may install it by invoking
python setup.py install
If you wish to install Omnifit on a computer on which you do not have administrator rights, you can instead use the command
python setup.py install --user
After installation completes you should be able to import omnifit in Python using the command
import omnifit
If this does not raise an error, omnifit has been successfully installed on your system!