Using Jokosher? Are you getting the error message “You do not have any LADSPA effects plugins installed” when attempting to add effects to an instrument?
rm -rf ~/.gstreamer-0.10/registry*
If you want more information, you can find it in the GStreamer FAQ. The bug is now closed, but this is still an issue in Ubuntu 10.04. Once Ubuntu updates their GStreamer packages this post should be obsolete.