that "suggestion" is for Xine, which is not MPlayer
run "mplayer -vo help" to see the possible video output options avaiblable to mplayer, the default X output is xv, is that what it's trying to use? try "mplayer -vo xv blah.avi" to use that specific output, if that is failing, try x11. you've not really said anthing about how you actually did install it though... does the "general" rule apply here? what version is it? provide more of the error for more help