The error you’re making is actually a pretty common one.
Basically, Rails automatically maps URLs for your scaffolds. So when you created the Posts scaffolds, Rails is mapping the URL routes for it. One such route is the URL for viewing a single post: /posts/(post_id)
So, when you’re entering the URL /posts/start Rails thinks you’re saying “Hey, give me the post with ID = start. So Rails complains that the show method can’t find a post with such ID.
One quick way to fix this is to make sure your config/routes.rb has the route for the start action before the scaffolding routes:
# Route for start action
map.connect '/posts/start', :controller => 'posts', :action => 'start'
# Default mapping of routes for the scaffold
map.resources :posts
Anyway, hope that helps.