Apparently, the problem is that when syslinux is not in the MBR there's nothing to point to where the thing we want to boot is located. Solution is to create the LiveCD usb version in Mac Os X:
1) Plug your USB flash drive into your Mac.
Open a terminal and run
diskutil list
Note the drive name of your USB, mine was /dev/disk1 but this number will differ depending on how many devices you have connected, look at the size to identify.
Open the Disk Utility and select your USB drive and erase it, leave it in msdos format.
2) Unmount the USB:
diskutil unmountDisk /dev/diskX
(Substitute X with your drive number obtained earlier)
3) Mark the partition as active
sudo fdisk -e /dev/rdiskX
(Substitute X with your drive number obtained earlier)
You will get a message saying:
fdisk: could not open MBR file /usr/standalone/i386/boot0: No such file or directory
Enter 'help' for information
Everything is OK!
4) Enter 3 commands
f 1
write
exit
Keep an eye on the desktop, if the usb drive mounts itself at any command that (last one included) then unmount the drive with diskutil unmountDisk /dev/diskX
5) Download syslinux
https://www.kernel.org/pub/linux/utils/boot/syslinux/syslinux-6.01.zip
6) cd to the syslinux-6/bios/mbr folder
cd Downloads/
cd syslinux-6/bios/mbr
7) Write the needed code to MBR
sudo dd conv=notrunc bs=440 count=1 if=mbr.bin of=/dev/diskX
(Substitute X with your drive number obtained earlier)
It will report number of bytes written as 440.
If you have an error, try unmounting the USB again or if there is no mbr.bin then check that you cd'd to the correct syslinux folder, it has to have mbr.bin inside.
7) Download the Ubuntu iso file
releases.ubuntu.com/13.10/ubuntu-13.10-desktop-amd64+mac.iso for 13.10
cdimage.ubuntu.com/daily-live/current/trusty-desktop-amd64+mac.iso for 14.04 (it just did a pre-release, works great on my 6-2 2013 year 13 inch MBA).
8) Install Unetbootin for Mac and make the Live USB following on screen instructions.
http://unetbootin.sourceforge.net/
It will take some time to finish...and it might seem that it is stuck - it is not!
When done then it reports that the USB will not be bootable on a Mac. (This is normal.)
(If when you open Unetbootin it complains about unknown source then hold Ctrl and click on the file and select Open then from the new warning you can select Open to actually open the program.)
9) Install Ubuntu
Restart your computer, hold the Alt key soon as it powers on and select the "Windows" option given, then choose Install ubuntu from the Unetbootin menu (you might have to choose it twice!). Follow instructions, I replaced Mac Os X with Ubuntu and let it deal with partitioning etc itself.
Source: ubuntuforums.org/showthread.php?t=2174630