Access your Cpanel access by http://yourdomain.com/securecontrolpanel. If your domain has not propagated, access the Cpanel via http://serverIP/securecontrolpanel with that domain's username and password.
Make sure that you are using the right Username and Password when the Password box appears on the screen. Your Username and Password is CASE SENSITIVE, so make sure to be exact here.
After your domain name has propagated, you can access your Control Panel via http://yourdomain.com/securecontrolpanel. Before your domain name propagates, you must access your Control Panel via http://IPADDRESS/securecontrolpanel.
After setting up the error pages in cpanel, you may need to adjust your browser settings to see this pages. If you are using Internet Explorer: Click Tools > Internet Options > Advanced and uncheck 'Show friendly HTTP errors'. If you are using a different browser, check the settings and look for something that relates to errors.
There is also a way to force IE to display custom errors no matter what. You just need to ensure the page is larger than 512 bytes. This does not include graphics, so make sure you add enough text to accomplish this (If you just need filler text, add a bunch of nonsense text in comments in the source code).
