I just downloaded DFT 55mm ver. 7 from dft's website. I installed it without problems on my imac 24" runinng osx 10.4.8 and FCP 5.1.2.
But when I oppened the filter and clicked the "option" button to enter my serial, nothing happened. hmmm.
I tried to trash my prefs and repair permissions but still nothing happened.
can it be that you have an earlier version and still have the old serial number in one of the fcp plugin folders?
don't have the edit machine running but it is either in the libray/application support/fcp plugin folder or in the fcp preferences in your home/library/preferences/fcp preferences/plugins folder.
delete (or move to trash but don't delete to be able to reverse the action) the old 55mm serial number and try again.
not sure if it works as i haven't updated yet, but worth a try.