Multiboot USB stick does not appear in boot menu

I had the same problem with my ASUS N56VJ-DH71 notebook. The solution I got from ASUS worked for me:

• Enter the BIOS setup menu by pressing and holding F2 when powering on
• Switch to "Boot" and set "Launch CSM" to Enabled
• Switch to "Security" and set "Secure Boot Control" to Disabled
• Press F10 to save and exit
• Press and hold ESC to launch boot menu when the notebook restarts
  (or you can enter the PC setting menu in windows. You should see "use a device")

You only have to boot another *nix OS (not the one with the stick you want to use) and use gparted or something like that to manually partition the USB stick (I had to first resize the current partition to make space). Create a new primary ext4 partition labeled "persistence". Kali is preconfigured to use that label so this is important. Mount the new partition and create in the root a file named "persistence.conf" and add just one line: "/ union".

That's all. I tried that and worked, you just have to boot kali in yumi using the persistence option.