I use Slamd64, too. I remember during RC2 xmms had a few plugin problems. Just as a test, I pulled the plugin I wanted out of an RPM from a SuSE 9.2 mirror (x86_64) and emptied it into (if I recall correctly): /usr/lib64/xmms
It worked fine!
It's not a decent solution for developers and distro releases, but it's functional for end-users who are more bothered about listening to music than intepreting error messages
Obviously in the long term, there may be some other problem here that we're not seeing - but give it a go, and perhaps paste in your configure options for XMMS (Slamd64 package, or custom compiled?) and the Flac plugin.