Where can I find the private key in a web wallet?
Click the settings cog in the top right (just to the left of logout).
Then select the Export Tab
Enter your Security ID and click Export.
That should download a txt file containing your Addresses & PrivKeys
In case you’d like to know more, here’s a nice guide explaining how to find your private key in the web wallet and in the windows wallet and how to import it to the one or the other.