Once you're at the recovery screen, press the Ctrl and D keys.

You'll see a screen that says " To turn OS verification OFF, press ENTER". Press enter to enable developer mode.

You will see a screen that indicates that OS verification is OFF. (If left long enough the device should beep at you.)

This screen is a security feature, as the developer mode doesn't have the usual security features. The screen is meant to help the normal user by guiding them through the process of disabling developer mode if they didn't mean to enable it.

To boot your device press the Ctrl and D keys again. Your device should boot automatically and will prepare your system for developer mode (This can take up to 15 minutes.