Does anyone have one of these? How is this used?

Yes, I have one and love it !

I got it sent to Australia from Country Kitchen Sweet Art.

You just roll out your fondant and place the acetate impression mat on top.
Either roll over with your pin or press all over with your hand (which is what I have resorted to) - sometimes I do it once the piece has been put on the cake - if I think there may be a chance of it distorting.

Look in my pics - there is a red bag where I used it