I'm sorry but I have removed the previous replies to you as they were totally and utterly wrong.
lol! you not a real programmer then Most programmer want to get rid of them not create them
Sorry, that made me larf.
You don't need to create an error to see the page, just navigate to it in your browser (http://localhost:300/500.html)
To find the files just look in the your_app_root/public folder and you will see a bunh of the default rails files (400, 500, index etc...)
As above, you don't need to do this but it answers the question you asked
Creating a 500 error is dead simple. Just reference a variable that doesn't exist in a view
<%= @something_that_doesn't_exist %>
Chuck that into an index.html.erb file and visit the page
run server in production mode (development mode will just get you the error output) then you will see the 500 page.
What you want and what you need are too often not the same thing!
When your head is hurting from trying to solve a problem, stop standing on it. When you are the right way up you will see the problem differently and you just might find the solution.
(Quote by me 15th July 2009)