Shortly after receiving payment confirmation from either Amazon or PayPal, you should receive an email from Humble Bundle. This email will link you to a key page with your Steam code.
If you did not receive an email, try using the Humble Bundle key resender:
https://www.humblebundle.com/resenderDouble check to be sure it is not being sorted into your spam box.
Once you've clicked on the link,
Click the Steam button at the top of your Humble key page to generate your Steam code.
Open the Steam application and log in. Click the "+ ADD A GAME…" button in the bottom left-hand corner. Select "Activate a Product on Steam..." When it asks for a Product Code, enter the Steam code from your Humble key page.
For detailed step-by-step instructions, check out Humble Bundle's support page.
https://support.humblebundle.com/hc/en-us/articles/204008710-Updated-Steam-Key-RedemptionLet me know if you're having any further issues.