Welcome to our community!
No worries, we'll get you sorted out :)
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 since you said you logged out last time by mistake.
The
Humble Support page on redeeming Steam keys should be helpful if you would like a step-by-step guide.
Hope that helps, please let us know how you make out!