its the 2 in the top right corner of the screen. once u press it. if you choose to you can double now by clicking on it, your opponent now has the choice. In this case it turns into a 4 then 8 etc..

My Friend, The Cube is NOT there , NOOO cube appears in the corner

There is a recent bug in the game where the doubling cube does not always appear.  Sometimes it will randomly show up after rolling, but most of the time it is not there.  The issue persists with both Firefox and Chrome for me.

Does everyone else have this problem?

I have the same problem, can't see the dubbing number - I can only respond if someone offers one. what is the problem?