I’ve been into this pain.. but having an alix helped me.
Basically the ISO image supplied for ZS is a VGA while APU require a serial.
I did it blindly, created an image on a stick, tested on a computer then insert it into APU. After boot I connected my laptop, assigned a static IP 192.168.0.10 and used the browser to connect to 192.168.0.75.
From GUI ( SETUP -> BOOT ) I changed the Console from VGA to Serial 115200 and then rebooted. Now the serial cable worked as it should and I was able to transfer the image.
I wanted to post my experience but beeing so cumbersome I wanted to investigate what files are changed on the image and then post a simple how to. Unfortunately I was more busy compiling the missing bits for APU ( LEDs, buzzer, GPIO, temperature ) .
You can get the APU missing drivers from here: http://jb68.com/pkg/
I also developed some LED scripts for APU GPIO with ZS that I am going to publish soon.