Thanks for reporting this!
I just checked the mplayer PBI in the AppCafe (version 1.0.r20120322_1_01), and the GUI is opening just fine for me (64-bit, XFCE). Could you try opening a terminal and running the command "/usr/pbi/mplayer-amd64/.sbin/gmplayer" to see if there are any error messages that appear?
I have installed the same MPlayer version. This is the output:
MPlayer SVN-r34821-snapshot-4.6.4 (C) 2000-2012 MPlayer Team
Icon 'mplayer' (size 16) not found or unsupported format.
MPlayer interrupted by signal 10 in module: unknown
- MPlayer crashed. This shouldn't happen.
It can be a bug in the MPlayer code _or_ in your drivers _or_ in your
gcc version. If you think it's MPlayer's fault, please read
DOCS/HTML/en/bugreports.html and follow the instructions there. We can't and
won't help unless you provide this information when reporting a possible bug.