If people need it, this is what I recall about the refund request process:
1.) Visit the project page and click on the creator's profile name (this opens their bio page)
2.) Click the "Contact me" button, and briefly explain the situation (i.e., pledged for Steam key, but game no longer offered on Steam).
It might also be worth asking if there is any chance to still receive a Steam key, as you are an original backer. I like to think that's a best-of-both-worlds solution, where backers have their original choice of the Steam version, but retail is limited according to their funding deal.