Exact same issue here :-|
Just bought Winamp pro for Android today on play.google, and wants to buy "Album washer" for Winamp to.
But I cant find it on Play or via the Link inside Winamp.
Its such a good offer so I hope I will be able to purchase it before the offer is no more valid.
Maybe the Album washer add on for Winamp pro hasn't been approved by Google Play yet?