Help With Barre Chords

This is a 1.75 minute video response from Matt Hammond on Nov 1st, 2008

Question:

"LipstickManiact says, "I have problems with barre chords. Just can't find the right position of my 1st finger. For example...if i play D major barre (C shape) 3rd string doesn't play and I have a similar problem with other chords. I can't understand and find the right position. I didn't find answer in lessons, because teachers just say lay down your 1st finger below the fret. Please help..just show me what to do with 1st finger exactly.""