After downloading IronRuby and JRuby to do some hacking over the Christmas period, I wanted to add them to my PATH variable on OSX. In Windows, this is fairly easy to find but is hidden a few dialogs down.
In OSX, you need to edit the .bash_login file in your user directory. A simple nano ~/.bash_login will allow you to edit the file (replacing nano is your own favourite editor).
To add IronRuby to your path add:
1) The path to your IronRuby bin folder.
2) Ensure you add the existing $PATH to the end
3) Export the result to the current PATH variable. Note, this is case sensitive.
To add jRuby you do it in a very similar fashion. However, because of priorities and how jRuby works, unless you want jRuby to be the default, add it to the end of the path export
You should now be able to see all your ruby interpreters.