wgl Extension

help!!!
when i tried
glSampleCourageARB = (AAA)wglGetProcAddress(“glSampleCoverageARB”); ,
the function is assign as 0x000000 , ?
however all of my the code for wgl Extension work well. And the card do support multi sampling (wglgetextensionstringarb indicted so)

The card i’m using is else syn 2

Thank in Advance