Can't find On the Rain-Slick Precipice of Darkness, Episode One in my library...

Had the same problem. In my case a simple Steam restart helped.

Maybe you should try deleting cache, reinstalling Steam or something?

Make sure your list is set to "All Games" and in search type "on the rain". It should appear as "On the Rain-Slick Precipice of Darkness, Episode One".

This is the store page to confirm you already have it. It must be say that in green:
http://store.steampowered.com/app/18000/

If not even closing, clearing cache, and re-starting the client doesn't help, contact Humble Bundle support. There is also a way to "reset" Steam, but also erases how you have configured Steam in your machine.

Problem is solved - I've activated only BTA key:) This is 2 separate keys non BTA and BTA, Thanks for help!