I had this issue as well. I had a very happy Lubuntu boot stick which I had used for emergencies so when it was time for an update I used unetbootin to install the latest version (14.10) and then ...
"no default or ui configuration directive found"
This was from a USB stick / PC combo that used to work. After wasting some time editing files from isolinux to syslinux (or something like that) the solution was a lot simpler, do a clean format using Gparted, before using Unetbootin, to either FAT16 or FAT32, I tried both and both worked.
I acceot this may not work for everyone, but it worked for me :-)