When you've set all the options in the Export Bitmap dialog as desired, you just need to press the Export button to export the PNG file.
Do note however that the Export Bitmap dialog does not close after exporting a graphic. It remains open and that can be slightly confusing at first as it can appear that it hasn't exported the graphic, but if you check the folder you are saving to, you should find a new PNG file. To close the Export Bitmap dialog, just click on the X button in the top bar.

