How do I install a torrent client on My Book Essential
You don't. Judging by a pic of the rear of a MyBook Essential I just googled it's just an external USB harddrive. I.e. no network port. Perhaps you could coerce a USB-> Ethernet dongle on the thing. But how would you access a command line in the first place, you'd need to get into the thing via a serial port or JTAG connection if that's even supported. And that's if a MB Essential is running Linux at all, which I don't know and can't be bothered to google.
I highly doubt that is runs a form of linux as it would be pointless. If it is a usb drive then it is just that nothing more.