The reason dd doesn't automatically make the disc image bootable from your flash drive is most likely because the image was not authored in such a way as to allow that. The problem is that the boot sector on your hard disk (MBR) is not the same as the boot sector on a typical CD image. The BIOS reads the boot sector on each medium slightly differently. If you tried to dd a Windows 7 SP1 x64 installation disc to your flash drive you would encounter the same problem. Utilities like UNetbootin and the Windows 7 USB Tool work around this problem by mounting the disc image, copying the contents to the flash drive, and properly installing a bootloader to the master boot record. In the case of the latter utility this always works flawlessly. Microsoft's utility can always properly configure the bootloader because the structure of Windows installation discs is predictable. UNetbootin, however, has to do more work. Not every Linux or BSD disc image is authored in exactly the same way, therefore while UNetbootin can install the Syslinux bootloader properly every time and can theoretically bootstrap any operating system supported by Syslinux, the reality is that it has to do some guess work to generate its Syslinux configuration. Most Linux distributions use Syslinux to boot their disc images and store their Syslinux configuration files in a standard location, therefore if you don't look too closely UNetbootin appears to work flawlessly every time. However if the aforementioned configuration is not present or not stored in a predictable location, as is the case with the vast majority of non-Linux-or-BSD disc images, Unetbootin has to do significantly more guess work to configure the bootloader, and its error rate increases exponentially. In short, it is not particularly surprising that neither of the methods you have tried thus far have not worked.
The information above begs the question, why does dd-ing disc images to a USB flash drive work some of the time? Like you mentioned, it is officially recommended on debian.org to dd the Debian Live or Debian Installer images to your flash drive. Also like Growled mentioned, the Arch Linux wiki recommends the same method. However both the Debian and Arch CD images are authored to generate an ISO-Hybrid image. Such an image contains a hybird partition table and the bootloader installations such that Syslinux will be loaded regardless of whether the image is written to a CD or USB flash drive. However this is an advanced configuration not often used on other boot discs or with other bootloaders. As far as I know neither GRUB nor any revision of the bootloader Microsoft ships with Windows (NTLDR) have ever been capable of such a configuration. Therefore it is actually fairly rare to find disc images that can also be directly written to a flash drive outside of the strictly open-source universe.
In conclusion, I don't have a bulletproof solution for you. I just wanted to explain the background of your situation and stem some of the factual inaccuracies this thread was trending towards. Although I don't use Hiren's Boot CD (doesn't that contain pirated software by the way?), it might be possible to boot that disc image from your flash drive using a different method. You may have some success installing GRUB 2, mounting the disc image as a loopback device, then chainloading it (disclaimer: here be dragons). Good luck.