You don't have to purchase it on Steam to access these, but you do need to have Fantasy Grounds in your Steam Library. To do this, you have to activate the product on Steam and enter in your existing FG license. Once it shows up in your Library in Steam, you will be able to accepts gifts or redeem DLC from Humble Bundle. Note that new licenses purchased from fantasygrounds.com are not automatically available for activation on Steam. We do this in batches every 6-8 weeks. I will likely do another batch next week after the Black Friday sale ends. If you need or want it sooner, you can contact [email protected] and David can give you a dummy key you can use to activate it. This won't actually be a valid key for Fantasy Grounds, but it will let you add it to your Steam Library for the purpose of adding DLC.
Here is the link on Steam explaining the process:
https://steamcommunity.com/app/25269...5530621472814/
Note that once you claim it and add the DLC, you can use the link on our store to sync your Steam account with your FG account without actually running FG from Steam.
Store > Steam Account Syncrhonization
https://www.fantasygrounds.com/store...eamAccount.xcp