Do you get any errors using the console?
With
Code:
/opt/mplayer/bin/mplayer "/Users/USER/Downloads/[Chihiro]_Sekirei_04_[h264][00B71E51].mkv"
and
Code:
/opt/mplayer/bin/mplayer -monitoraspect 1.6 -vo macosx -ass -embeddedfonts -vf-add screenshot "/Users/USER/Downloads/[Chihiro]_Sekirei_04_[h264][00B71E51].mkv"
?