I have recently brought some sheet music and in the tabulature key it says that to perform an unpitched mute you dampen the strings with your left hand. But which frets to you dampen when it is an open string that is to be pitch muted ?

any fret, just lightly rest one of your fingers on it to mute the string.

EDIT: I'm thinking of something else, sorry.

Percussive mutes I suppose are slightly different. You don't want to dampen them somewhere that will produce a harmonic. Play around, you'll find some good places to do it.