Love that you are interested in Hero Academy! We love the game, but we don't directly sell Hero Academy.
If you've got a valid Hero Academy Gold Pack, you can redeem that on our website, but those packages do not include a Steam Key. When we've given these codes to a vendor to sell there are Steam Keys that go with them. Maybe they forgot to send you their key?
You will need to talk to the people you purchased the game from to get your Steam key to activate the game. (It might be that the place you bought the game from just intended to sell you the upgrade and not the game itself. )