the zigbee part is easy. Start the SmartThings pairing from the app and then plug it in. It should find it. Verify it works by plugging a lamp or something into it, turning it on and off from the app and making sure the power draw changes and the device turns on and off.
The z-wave part can be tricky. From the linked thread:
"Second: The Z-Wave repeater
Once you have the Zigbee Plug added into SmartThings you can go back to add the Z-wave repeater. In order to do this, first start the pairing mode by selecting ‘Connect New Device’ within the Marketplace in the SmartThings app. Once the hub is searching for a new device, hold down the button on the front of the Iris Smart Plug until the light begins to blink, this should only take a few seconds. Some users have reported than holding down the button doesn’t work and instead they needed to tap the button 8 times rapidly in order to get it into pairing mode. Once the light is blinking SmartThings should discover the repeater as a ‘Z-Wave Device’. This is the proper devicetype for the repeater"