I was creating a bootable USB flash drive (of GParted) using Tuxboot on my Windows 7 x64 computer. I kept receiving the error during the "installing bootloader" phase: the file does not have a program association with it for performing this action. The USB drive would not boot either, and I would get an error that "bootmgr is missing".
I found a recommendation to run Tuxboot as administrator, which did not help me (but maybe it will help you).
I resolved the "bootmgr is missing" error by formatting the flash drive as FAT32 and then running Tuxboot again. I still received the error message, but my recovery drive was created successfully.