I'm not sure how to setup in app purchasing for Steam.
I've already read the following:
So far I have completed:
Also I've implemented Google Play IAP without even using Game Sparks and it was quite easy.
You'll need to do most of the configuration on the Steam side. Prices are set on Steam. You'll need to refer to the Steam documentation for more information in relation to this. Once your goods are configured you'll simply need to set up your Steam integration on Gamesparks and give each Virtual Good the appropriate Steam ProductId so that it can be awarded once it has been validated with the SteamBuyGoodsRequest. If you have any further questions then just let us know.