"source/ocio" in the example cmake command needs to be replaced with the path to the OCIO source directory on your machine ("/home/sigi/Downloads/imageworks-OpenColorIO-a4dc9cd" in your case, pictured in your screen grab).
So you should:
1. Create an empty build directory somewhere and cd to it
2. From this directory, run cmake with the command:
cmake -D CMAKE_INSTALL_PREFIX=<where OCIO should be installed> <where the OCIO source lives>
This will configure the project for building within your current directory when you later run "make", and install to the install directory when you then run "make install"
Hope that helps.