Have you correctly entered all of the fields listed in humancontrollers post? (classname, radius, targetShaderName (origin should be automatically entered, as should classname)).
Just copy what he's written exactly into a misc_light_flare entity, and it should work. You'll get a red flare if you use "models/weapons/prifle/flash"
To get other flares you have to change the path to a different lens flare texture, i.e. "textures/niveus/flare_red_01" is a red flare from niveus.