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You should get an email from Humble Bundle (check your junk mail box). This has a link to the downloads. On that page there is a button that says 'get on steam' click this. Then click the button to generate the steam key code. Then activate that key through steam.

It takes a little bit, you'll get an email with a link to your game purchases, then you can activate them on steam or download the DRM-free version. It took about half an hour to get my email this time, I think it's because it just started and the massive influx of people buying it right now.