thanks for asking.

The Mplayer Behaviour is not broken. In fact, it didn't change at all 
in Preview 10b. And since Remote Buddy is already using the version 
you tried to install from inside Remote Buddy's bundle (which, btw, 
you shouldn't do), it also acted correctly by bringing up the dialogue.

If the Mplayer Behaviour does not turn up in Remote Buddy's 
Preferences, Mplayer was not found through Launch Services (a system 
level service by OS X).

You can use LSRefresh from here:

http://www.iospirit.com/index.php? 
mode=view&obj_type=blog&obj_id=2&o_blog_dmode=view&o_blog_entry=79

to refresh / add an entry for Mplayer in your LS database. Just fire 
up LSRefresh, select "MplayerOSX.app" in the dialogue, let LSRefresh 
update your Launch Services database and restart Remote Buddy 
afterwards and the problem should be gone.

You can also try to use OnyX to entirely rebuild your Launch Services 
database. However, I never did the latter, so no guarantee for that 
way from my side.

Hope that helps.

Best regards, 
Felix