Good advice from Krakob.
As he said, the same page where you downloaded the game will now show the Steam Key,
click here and select "Slime Rancher".You can also try the
Humble Order Resender which can be used to resend the download links for your Humble purchases.
The
Humble Support page on redeeming Steam keys should be helpful if you would like a step-by-step guide, as stated by default Steam will keep the game updated automatically.
Hope that helps, please let us know how you make out!