Basically you should turn it into an .iso then burn it at 2x to a DVD or have Boot Camp Assistant use the .iso to make a USB W7 installer drive. From what I understand Windows only see's a .iso as a disk image.
When you use the Boot Camp Assistant to make a W7 installer of an iso it will make the .iso bootable at the same time. Other wise you can do it manually with an iso bootable application.
Yahoo/Google would be your best bet for it, sorry I don't know any off hand.
Basically you may not have a supported model. Here's a link to a possible work around. You might try reading the whole post for different way's of doing it. There's like 5 pages so be patient. Some of the earlier Mac's can't boot via USB is one of the issues.
Make sure Lion is updated all the way as I believe Apple added the USB W7 installer option in 10.7.2?
Worse case scenerio you may have to just burn it to a disk, but it has to be done in a way that makes it bootable, simply dragging the files over doesn't work.