DragonKing: You can double from the Game menu or using <Ctrl-D>. I don't do either of those - I just click the doubling cube, lolol. You can also click it to accept a double when it's offered. Decline by using the icon on the toolbar.

If the game you're following shows no play then enter the dice and gnuBg will tell you that it's no play. ;-) If you don't know the dice - such as with the game records here :-, :-/, :-(( - then just make up a dice roll that you know won't work.