The $material is defined outside the

"global proc transSliderButton( string $HUD )"

So it will not work ...

in the global... you must declare it

so the script will go like that

"


global proc transSliderButton( string $HUD )
{
string $material = "lambert2";

undoInfo -swf true;

float $value = `hudSliderButton -q -v $HUD`;

setAttr ($material + ".transparency") -type double3 $value $value $value ;
}

hudSliderButton -s 5
-b 5
-vis 1
-sl "Trans:"
-value 0
-type "float"
-min -0
-max 1
-slw 50
-vw 50
-sln 100
-bl "Close"
-bw 60
-bsh "rectangle"
-brc "headsUpDisplay -remove HUDTransSliderButton"
-sdc "transSliderButton( \"HUDTransSliderButton\" )"
HUDTransSliderButton;



I hope i helped