From your original post:
"I've got an eeepc 701SD with backtrack installed on the 8Gb drive"
It sounds like you've got the CD version installed and not the USB (Extended Version).
The USB Version has the kernel sources included.
From the wiki, here is the download location for the kernel.lzm file:
I've never added the kernel sources this way, but I would assume that you would place the kernel.lzm file into the /BT3/modules folder and then reboot.
Then try my instructions again.
Hope this helps.