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.
Note: Custom error pages do not show up on temporary access url's (url's IP address, server name etc.) for your account.