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:

http://www.mongodb.org/display/DOCS/Rails+3+-+Getting+Started

also tried this one: https://github.com/RailsApps/rails3-mongoid-devise

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?