x-mirror, weight paint question

I can’t get the mirror to work while weight painting.
I used x-axis mirror while extruding the armature, then I pressed the x-mirror button in the weight paint menu. I also named everything like in the tuturial with a .l or .r after it for left or right side. please help. here is a picture.

have u checked the wiki? http://mediawiki.blender.org/index.php/Manual/Weight_Paint