You install the OS on a TF (Micro SD) card using your desktop computer and then put the card in the ZeroPi. After booting, you can connect through the serial port or ssh via Ethernet or WiFi, depending on what you have set up.
Insert the SD card into another machine, add a boot and filesystem partition, install OS onto it, mount the filesystem of the SD card, chroot to it, and install sshd.
More at http://wiki.friendlyarm.com/wiki/index.php/ZeroPi#Configure_...