CorelDraw 11 PNG Transparent Background

Hi,

i'm having problems with export option in corelDraw 11, i'm trying to export a .Png file with transparent background, when the "convert to BitMap" dailog box opens up, the "tranparent Background" option is inactive.

in the export dailog box, checked the "selected only" option

i tried everything, check anti-alias, and choose RGB as color mode and gave 300dpi res, still, the "transparent Background" option is not active ! any thing i missed out guys, kindly adivse

kind regards

arfath