[tor-relays] setting up a Tor exit node

I want to use Tor browser bundle to set up a Tor exit node in vidalia, I click "set up relay" -> sharing then there are some options, I checked "Relay traffic for the Tor network(exit relay)" and then click "ok" however, it doesn't work, and the exit relay option is automatically unchecked, while the option "relay traffic inside the tor network (non-exit relay)" is checked how can I set up a Tor exit node? thanks!