How do I give the ENTER keypress?
You can't just "print" a key like that. You use print for sending text.

When you use println it sends an enter key along with your text, so you can do :

Keyboard.println("ls -al");


and it will send the text ls -al and press enter for you.

If you want to do it in the 'raw' way you will need to press and then release the key:

Keyboard.press(KEY_ENTER);
Keyboard.release(KEY_ENTER);