After a successful installation of FAST for SharePoint 2010, you need to run the FAST configuration wizard. If you had FAST installed on a system earlier and uninstalled it, you may have some issues trying to run the post-install config wizard. My wizard kept failing with a cryptic error message in the log:
To be able to run the wizard successfully, make sure that you’re running it as Administrator. If that’s the case and it’s still failing, then try deleting the FASTSearchAdministrators group which was created during the first install. That did the trick for me and the wizard ran successfully.