beginner question: how do I figure out why an app won't start? (RubyMine)

I'm building a new Ruby app with RubyMine. I built a basic app (just a homepage) and it is running fine. Next step is to get the app talking to MongoDb. I'm using this tutorial:

also tried this one: GitHub - RailsApps/rails3-mongoid-devise: An example Rails 3.2 app with Mongoid for data, Devise for authentication.

The problem is, after running through the tutorial and starting my app, nothing happens. Mongrel boots up fine:

D:\Ruby193\bin\ruby.exe -e $stdout.sync=true; $stderr.sync=true;load($0=ARGV.shift) D:/Ruby193/lib/ruby/gems/1.9.1/ gems/ruby-debug-ide-0.4.17.beta8/bin/rdebug-ide --port 8051 -- D:/Dev/ rails3-mongoid-devise/script/rails server mongrel -b 127.0.0.1 -p 2999 -e development Fast Debugger (ruby-debug-ide 0.4.17.beta8, ruby-debug-base 0.11.30.pre3) listens on 127.0.0.1:8051 => Booting Mongrel => Rails 3.1.1 application starting in development on http://127.0.0.1:2999 => Call with -d to detach => Ctrl-C to shutdown server

But when I browse to http://127.0.0.1:2999 I get a 404. So something isn't happening, but there's nothing coming back from mongrel to tell me what is going on. There's a 'Server Development Log' tab but all that has is (when running the Mongo project):

   MongoDB logging. Please note that logging negatively impacts performance and should be disabled for high-performance production apps.

When the app loads successfully there's GET requests in there etc.

So I'm stuck. I have an app, it's building fine, it seems to start fine but there's nothing there when I browse to it. The routes.rb says ` root :to => "home#index" ` and there is a index.html.erb. So it certainly looks to me like it should be loading an index page. It isn't so there must be something wrong, right? What's the next step to figure out what's wrong here?