Have you read the Wiki at www.dovecot.org
and looked carefully at the Postfix documentation? I know that there's at least one Dovecot + Postfix guide out their on the net if you Google for it. If that hasn't helped, please tell us exactly what isn't working, as your question is very broad.
Also, I think FC ships with a 0.99 version of Dovecot. You *may* want to try upgrading to the latest 1.0-alpha release. Dovecot has been under rapid development in the past several months, and the 1.0-alphas are a near complete rewrite that handles a lot of stuff better. I still use 0.99 in production sucessfully, though, so YMMV.