Yeh, I tried running that a couple of times as my normal username. Interestingly, the gst-register program doesn't seem to be in root's path on my machine.
Also, the SuSE rpms don't seem to have come with any mpc support - when I try and add an mpc file to amaroK's playlist, it tells me that the media could not be played. If I change engine to arts, the same files play fine. Output of $gst-inspect | grep *mpc* gives:
$gst-inspect | grep mpc
typefindfunctions: audio/x-musepack: mpc
typefindfunctions: application/x-apetag: ape, mpc
Is there any way of manually adding plugins that you know of?