It would help to know what you downloaded and from whom (provide a link).
There's this site:
Flashback Trojan - Detection, and how to remove - http://www.f-secure.com/v-descs/trojan-downloader_osx_flashback_k.shtml
which has a link to a tool that you have to run as an application.
There's Apple's site where you run an updater (wither independently or through Software Update) which is also supposed to handle things.