Finding the session key in Safari

First on all, if you don't see the Develop menu Safari:

Press Cmd + , to open the Preferences
Go to the Advanced tab
Check "Show Develop menu in menu bar"

Now to find the session key:

Go to the Humble Bundle website
Make sure you are logged in
Press Cmd + Shift + I to open the Web Inspector (or from the menu bar select Develop > Show Web Inspector)
Select the Storage tab in the Web Inspector
On the left pane, select Cookies
You should see the list all cookies stored for Humble Bundle website. Double-click on the row with name _simpleauth_sess
Copy the Value in the popup window

For a visual guide on steps 4 to 7, see this picture.