You'll need the "flash cable," which looks like a standard MicroUSB cable but actually provides 1.8V TTL serial to the Peek (the Peek doesn't support USB but rather exports its serial RX/TX ports over a MicroUSB connector).

Peek themselves sold one for a while and you might be able to get an official one - otherwise they're not hard to build.

See http://elinux.org/Peek#External_MicroUSB_Charge.2FUpgrade_po... for more information.