I used the standard v8.1 i386 memstick image, dd'd directly onto a stick, and it works fine with my Intel N270/ICH7/945GSE-based box (Aaeon 6831). Can you see if the boot messages show the USB hub detected? Look for a pile of messages like this:
uhci0: <Intel 82801G (ICH7) USB controller USB-A> port 0xfe00-0xfe1f irq 23 at device 29.0 on pci0
usbus0: <Intel 82801G (ICH7) USB controller USB-A> on uhci0
I would check your BIOS settings, and double-check the stick image after that. Do you know how to go from the installation menu to the "Fixit" console? You can use this to check the boot messages as well.
The last time I looked at a FreeBSD memstick, the install files were just in a directory on the already-mounted memstick. So the choice would not be USB, but "File System - Install from an existing filesystem". The files are in a subdir named for the version, like "8.2-RC1". Can't recall whether you have enter the path, but it would just be /8.2-RC1.
Just tested this, from the real memstick image, NOT something created by unetbootin.
It appears to run from memory, the filesystem isn't mounted until told. When it gets to Choose Installation Media, selecting USB comes up with "Message: Using USB device: da0a". Then it runs. If the memstick isn't found, go to the Options screen, highlight Re-scan Devices, and press space (Enter does nothing).
I would guess that unetbootin isn't creating the partition layout the installer expects, but don't know.
If you don't have a FreeBSD install to create the memstick, you can use Linux's dd or try this: http://www.chrysocome.net/dd. (Untested by me, but it should work.)
The installer may be expecting a certain partition layout on the install media, and unetbootin and grub4dos may or may not create the right kind. Have you tried the actual, real, unsimulated, nonduplicated, genuine FreeBSD memstick image installed on the USB stick with dd?
It appears that your problems are caused by using Linux tools to copy a FreeBSD image to a USB stick (or at least that's how I read it, there may be translation problems). Since FreeBSD Is Not Linux, this is not guaranteed to work.
How many times did you try rebooting? Sounds silly, but after my first reply to you I experienced the same problem and "fixed" it by rebooting.
When I tried to use a USB stick to go to the Fixit console, it could not find any USB devices. This was on a Atom N270-based box, with a USB stick containing FreeBSD v8.1 i386 memstick, created with dd on another FreeBSD box. It did not recognize any devices even after a "rescan". So I powered down, rebooted, and the USB was recognized on the second try.
I have no idea if it was a hardware or software fault, and I wasn't able to capture any boot messages at the time. :\
Download dd for Windows from http://www.chrysocome.net/dd. Extract on your Windows system. Copy dd.exe to the desktop.
Download the FreeBSD memstick image onto the Windows desktop. In this example, it will be FreeBSD-8.2-RC1-i386-memstick.img.
Connect the USB memory stick to the Windows computer. Determine the drive letter Windows calls the USB memory stick. In this example, it will be F:.
Use dd to write the image to the memory stick (paths may have to be adjusted if you use Vista or Windows 7):
[cmd=">"]dd.exe if=FreeBSD-8.2-RC1-i386-memstick.img of=\\.\f: bs=1M[/cmd]
Disconnect the USB memory stick, it's ready to use.
..or maybe not. I'm seeing a "Boot error" on the stick I just made with the procedure above. Not sure why yet, and that stick had some experimental fdisk and gpart stuff a little earlier. I'll follow up later.