This is a little bit dated but the way to go is here:
The only basic difference when running in emacs is you want to start
your server (script/server) from inside a shell window like this:
$ cd your_project_root
I've used this method with ruby 1.8.7 / rails 2.2.2 and it definitely
works. Unfortunately for me, I'm working with ruby 1.9.1p0 and ruby-
debug hasn't quite made it there yet.
Not sure what you're using for emacs but I've found aquamacs (cocoa
build on emacs 22.3.1) to work quite nicely on OSX. What are you
doing to get your "rails-mode" functionality? I've used rinari and am
currently working with emacs-rails - both located by google.
I've needed to noodle emacs-rails a bit to get it to do something
reasonable with menu driven rspec variants.
Let me know how it works out for you.