is this a version of Mplayer embedded into / bundled with MythFrontend?

Or a regular version? If it is a regular version, you might be using 
an outdated version. From the FAQ (on a related issue):

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MPlayer OS X 2 does not appear in Remote Buddy's menu. (FAQ015E0162)

You are very likely using an outdated version of MPlayer for OS X. 
Older versions of MPlayer identify themselves with a different Bundle 
Identifier than Remote Buddy is using to detect an MPlayer install on 
your system. Please install an uptodate version of MPlayer (to be 
found on the MPlayer HQ site - the version you find on the dedicated 
MPlayer OS X site is outdated) in order to receive support for it 
through Remote Buddy. 
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Best regards, 
Felix