[eluser]TheFuzzy0ne[/eluser]
If you want to redirect and use an error message, there are two options available that I know of.
The first - my favourite- is to use flashdata to pass a message over to the next page, and display the message. This requires you to have the [url="http://ellislab.com/codeigniter/user-guide/libraries/sessions.html"]session[/url] class loaded, and your user will need to have cookies enabled.
The second, is to simply redirect, and add an extra segment to the URI to let the controller that's receiving the redirect know there was an error, and display it. This only works if the controller you're redirecting to accepts a strict, set number of parameters. If your controller accepts a variable number of parameters, you might want to [url="http://ellislab.com/forums/viewthread/106502/"]check out the two functions I published today[/url], which will aid in passing a message over in the URI.
All the best!