So you've downloaded the swi-prolog package from http://www.swi-prolog.org/Download.html and you need to run it.
It runs in the terminal but you need to run, opt/local/bin/swipl
If you have a folder with your file in, e.g. Download/myfile.pl then why not navigate to that folder in terminal before running swipl. That way you can edit and create files easily in your editor of choice and load the file easily in SWI-prolog.
By the way, to load a file type, ['my file.pl'].
You can, if you want, run the PCEemacs editor. You'll want to do this sooner or later.
To run this you should type, emacs.
It may ask you to load X11. Just follow the instructions.
