I believe this is a known issue with external monitoring of PAL DV video.
Did you try: Modify>Interpret Footage > Field Order...?
I'd recommend checking the field order settings inside the black magic design control panel. I don't use a mac though, but in windows this setting is found by going to the "control panel". In windows the black magic design control panel section has a lot of settings that can be adjusted and if my memory serves me correctly field order is one of them. I had this problem occur a long time ago if my memory is serving me correctly and I believe I recall fixing it this way. I am remembering this issue from quite awhile ago though, so I could be recalling the settings you can chose from incorrectly although I'd say it's worth looking into.
However you may want to check this thread out too it talks about reasons decklink cards can sometimes cause field reversal (under certain scenarios)