STTZ  We're talking about the keyboard keys and not the buttons in the player window, right?

In the player window you need to press alt to skip 10s. If you're using the keyboard, left-arrow/right-arrow skip 10s and that cannot be modified by a modifier key.

Though, MPlayer always skips to the next keyframe and so you never get exactly the time you tell it to skip. In the file I just tried it e.g. skipped about 15 seconds on each press. If you have a file with only very few keyframes, the skip duration could be longer. There's currently no way to change that, though.  
(Version rev11)

STTZ  The left/right arrow keys skip 10 seconds.

left/right: skip 10 seconds

up/down: skip 60 seconds

page-up/page-down: skip 10 minutes  
(Version rev11)

STTZ  I haven't heard of frequent crashes with MPlayer OSX Extended. Please send me the log messages you see before your computer crashes and I'll look into it.  
(Version rev5)

STTZ  The first time mplayer open, fontconfig needs to build it's font cache. This can take up to a minute but has to be done only the first time.

Fullscreen controls have been requested several times and I'll try to find a good way to implement it.  
(Version rev5)