When you first sign up for an account, you'll see a notification box that allows you to reveal your private key:
Enter your password and hit reveal key. You can copy the key or write it manually for the highest security.
If you already have an account, you can reveal your private key at any time by navigating to your account Settings from the menu button at the top right.
Once on the settings menu, scroll down until you see "Private Key":
Enter your password in the space provided and click "Reveal".