What OS are you doing this in?
If you are running Vista then you need to run the cmd.exe as an administrator (right-click->Run as Administrator). If not, the modifications will fail but not in anyway the BackTrack script recognises so claims it was successful.
also when i type in L: for my USB drive nothing comes up just L:\> i saw this video tut and when he did it,it showed him what was in his drive.
Apologises if I'm stating the obvious but I've seen people make similar mistakes when following tutorials and videos for things that are new to them; is
your USB drive assigned to L: ?
Finally have you set you BIOS to boot from USB first? Also try different USB ports, some machines are configured to only boot from some of the available ports.
Hope this is of some help, good luck